Here is an excerpt from Plato Book V:
SOCRATES: “Tell us this: does someone who knows know something or nothing?” You answer for him.
GLAUCON: I will answer that he knows something.
SOCRATES: Something that is or something that is not?
GLAUCON: That is. How could something that is not be known?
SOCRATES: We are adequately assured of this, then, and would remain so, no matter how many ways we examined it: what completely is, is com- pletely an object of knowledge; and what in no way is, is not an object of knowledge at all?
GLAUCON: Most adequately.
SOCRATES: Good. In that case, then, if anything is such as to be and also not to be, wouldn’t it lie in between what purely is and what in no way is?
I understand that the concept of “is” is related to something that exists or is true. But what does “being” mean in this context?
There is also this statement later on:
SOCRATES (in the context of belief): Do you know what to do with them, then, or anywhere bet- ter to put them than in between being and not being? Surely they cannot be more opaque than what is not, by not-being more than it; nor clearer than what is, by being more than it.