Hume was deeply skeptical of metaphysics. For a non-philosopher this includes topics such as being, essence, substance, space, time, the self, and causation.
For example: in A Treatise of Human Nature Hume argues that we have no innate concept of Cause and Effect (C\E) and that C\E is not rationally known but instead we develop the concept of it through the near universal juxtaposition of two events.
For Hume our knowledge of metaphysical subjects is not rational, but is caused by non-rational mechanisms in the mind and we do rely on them, especially time, space, C\E, and the self. But they are not uniquely privileged and thus cannot provide certainty for our beliefs. Also,they may properly be subjected to skeptical critique.
This has significant implications for both science and religion. And the religious
authorities of his day saw his work on causation as an attack on the foundations of religious belief. That was before he published his work on miracles, or his famous polemics against popular religion.
Kant read at least books I & II of Hume's Treatise(in translation) but he was
not entirely persuaded and wanted to save metaphysics. At least a little bit. Kant argued in The Critique of Pure Reason that for us to have an experience, the mind must impose certain structures on incoming sensory data. These impositions are "known" a priori (before experience) and thus are metaphysical, and to an extent privileged. There aren't many of these beliefs but they include time, extension (space), the self, and Cause and Effect. Kant also believed in universal and immutable laws, something Hume denied.
Speaking broadly: Hume tried to trash metaphysics while Kant tried to save parts of it. Kant's theory of a priori truths --especially his theory of synthetic a priori truths-- is fundamentally incompatible with Hume's more empirical approach. Hume is traditionally seen as the more skeptical of the two.
Also, Hume, before Kant, argued that we cannot get to the object-in-itself, even if he did not use that terminology. For Hume the mind operates on impressions and ideas. They start with sensory perceptions, but there is no guarantee that this are correct or accurate.
Here is an exert from Hume's Treatise Book I, Part II, Section VI: The Idea of existence and of external existence.
"nothing is really present with the mind but its perceptions or impressions and ideas, and that external objects become known to us only by those perceptions they occasion [...] it follows that tis impossible for us so much as to conceive or form an idea or anything specifically different from ideas and impressions [...] the farthest we can go towards a conception of external objects ...is to form a relative idea of them without pretending to comprehend the related objects."
You aren't a lone in struggling with Kant. He was a terrible writer. I once heard a Kantian say that every English translation of the Critique sucks, because the German sucks. Despite that, make sure you are reading a decent translation. Guyer/wood is recommended or Pluhar. Kemp Smith is considered dated at this point, but his translation was the standard for at least a generation or two of Kantian scholars.
Reference to secondary literature may help. The Blackwell Companion to Kant may be a good place to start. Or the Stanford Encyclopedia of Philosophy