I am reading a secondary source that has a section dedicated to Hegel. It says “For Hegel, the whole of history is such a dialectical process—one that is driven by Geist returning to itself, having “emptied” itself into time.” What exactly is meant by Geist returning to itself? How is it possible for something to return to itself? That implies something can leave itself in order to return back to itself. How does that even work exactly? Can someone give me other examples of things returning to themselves? And also, what does it mean for something to empty itself into time? Please understand, I am a novice when it comes to philosophy, so please spare me philosophical/Hegelian jargon if possible. So in other words, don’t give an explanation as if I already understand the topic I’m asking about.
This passage (you are quoting) takes something from the general structure of Hegel's Encyclopedia, and mixes it together with a quote from the end of Hegel's (early text) the Phenomenology of Spirit.
Let me start with general things about Hegel's system.
Hegel starts with Logic, which develops all the categories of thinking considered as pure thoughts, without reference to anything external. So Logic discusses basic philosophical categories (being, nothing, existence, cause, effect, finite, infinite, one, many, etc.) and tries to show that each one, if taken in isolation, is self-contradictory. Then, Hegel attempts to show that we can systematically derive the entire system of all philosophical categories based on how they contradict themselves. He does this through what he calls the Dialectical Method
The Logic transitions into the Philosophy of Nature, which dialectically develops all the categories of thinking, yet understood in terms of the "thought of externality." For example, instead of starting with Being, Hegel starts with pure space (space is "externality"). Instead of turning into Becoming, Space turns into Time (time is the movement of becoming with reference to externality. It's not a "pure thought," but the thought of becoming occurring within nature).
The philosophy of nature develops the concept of life, and life itself already shows signs of implicit mental activity. This leads to the final portion of the Encyclopedia, which is The Philosophy of Mind/Spirit.
Now Logic was already engaged with mind, because the Logic is a science of pure thought. So in this sense, the Philosophy of Mind is a return to itself from out of the externality of nature. Yet the Philosophy of Mind examines what we might call "embodied thought", or thought considered as it develops itself through the concrete activity of minds. This book of the Encyclopedia will end by discussing different attempts to give expression to the idea of "absolute mind/spirit", including various forms of representational art and the content of various religions. Yet for Hegel, absolute spirit is philosophy, which is in fact "thought thinking itself," what Aristotle described as God. Philosophy is the system itself, which Logic expresses as a science of various logical categories, nature as thinking that's external to itself, and mind/spirit as thinking that has returned to itself out of nature and attempted to understand itself concretely as its own activity.
Logic: pure thoughts, thinking about thinking.
Nature: thoughts but of nature, of external things, thinking about thinking about things outside of thinking.
Mind/Spirit: thoughts about mind, or thinking about thinking. Yet different from Logic because mind as "evolved" as it were out of nature, and thus is a return of mind to itself out of nature.
Answer to your question
So then, you asked about history. Importantly, Hegel understands the history of philosophy to have a certain affinity to the logical development of the logical system itself. For example, Parmenides puts "being" as his first principle, Buddhism puts "nothing" as its first principle, Heraclitus puts "becoming" as his first principle, and so on. So we could see the history of philosophy as a history of various attempts of the mind, or thinking, to know itself.
Moreover, he understands world history to be a progressive attempt to realize the concept of freedom concretely.
Your secondary source is likely based on a popular misreading of Hegel, that attempts to theologize Hegel's "absolute spirit" into some kind of metaphysical spiritual God-entity that guides the flow of history and attempts to realize itself through human activity. Based on this misinterpretation, some scholars think that Hegel believed in some kind of spooky "world spirit" that guides the rational course of history. Hegel does sometimes speak this way, but he likes to wax theological, using ideas borrowed from religion to help us picture the philosophical process of development. Yet you will never find Hegel develop this concept of the spooky world spirit, or the absolute in a rigorous philosophically defensible way! In fact, Hegel explicitly rejects the view that all of reality can be understood to be parts of some larger "absolute," claiming that this metaphysical position is nonsensical and undialectical.