Checking out source code
The term "Check out" is taken from the metaphor of a public library where one can "check out" a book. A checked out code, is locked from others to check out the same code, whilst creating conflicts. When you are done with your edits and enhancements, you return the code back (check in).
Please read
[
this article] to understand further.
Functional Specifications Document
A functional Specifications Document (or "Specs"), is a document where the product / project manager describes the functions that the program, you are working on, should perform.
See also:
this article.
Unit Testing
Unit Tests are small parts of a whole (program), usually a single function or a short pice of code, which can be tested separately and automatically. See:
this article.