Aspect refers to when, in time, is the action represented by the verb, done.
Is it happening, as in "I'm walking", or it happens in general but not necessarily now "I walk", Linguists try to understand language by classifying verbs according to how much extra information is needed for them to express what they are designed to express.
I learn French for instance, cannot be shortened into I learn. It needs a complement. It also needs a time. I learn French every four years. More appropriate is "I'm learning French", implying that learning is taking place now.
So action in progress, action accomplished, action not tied to specific time, those are aspects of verbs. I hope this helps.