The unit of analysis is the entity that you're analyzing. It's called this because it's your analysis (what you want to examine) that determines what this unit is, rather than the data itself.
For instance, let's say that you have a dataset with 40 students, divided between two classrooms of 20 students each, and a test score for each student. You can analyze this data in several ways:
Knowing your unit of analysis is helpful, because it helps you determine what kind of data you need. The other piece of this puzzle is whether you need macrodata (aggregated data) or microdata.
So what is the difference between macrodata (aggregated data) and microdata?