That's not important. Of course it's normal you ask to get started on unit testing, but I repeat it, it's not important.
The unit is something along the lines of :
- the method invoked by the test. In OOP this method has to been invoked on an instance of a class (except static methods)
- a function in procedural languages.
But, the "unit", function or method, may also invoke another "unit" from the application, which is likewise exercised by the test. So the "unit" can span over several functions or even several classes.
"The test is more important than the unit" (testivus). A good test shall be :
- Automatic - execution and diagnostic
- Fast - you'll run them very often
- Atomic - a test shall test only one thing
- Isolated - tests shall not depend on each other
- Repeatable - result shall be deterministic
