Do you remember the words for good (bona) and easy (facila)?
These two words end in an a because they describe things (they are called adjectives). All adjectives in Esperanto end in an a (just like all nouns end in an o, as we previously saw).
But let's consider these words that end in an a. Another way to put it is: you could think that a means related to or pertaining to. Thus bona means relating to goodness, pertaining to a group of good things, or simply put, good. And facila means relating to facility, pertaining to the group of easy things, or simply put - easy. Thus our a seems to indicate possession or pertinence.
Now, you have these words for vi (you), mi (I) etc. What would happen if you added an a to them?
For example, you take vi (which means you) and you add a and you get via which would mean related to you, pertaining to you or simply: your!
If you want to make words like mi, vi, ĝi, etc. possessive, you add a to them, to make them into mia, via, ĝia.