.gitignore file tells git which files you do not want included when you upload to your repository. For example fiels with personal info, or just things irrelevant to other people who may contribute.
They can live multiple places, so you can exlude things globably, or per-project.
It's a little bit fiddly to make dot files on windows, but you can do so from the command prompt. Either:
NUL> .gitignoreto create a new file.
REN [existing file] .gitignoreto rename a file you've already made.
To open a command prompt at your current location in Windows Explorer enter
cmd in the location bar and hit
Enter. You can jump there quickly with either
# comment *.ext file/path/ directory/ [Cc]ase
So for Unity game you probably want to ignore those .pdb files.
# Visual Studio / Unity *.pdb # Temp files *~
If you want to, as a base, ignore everything, then specifically tell it what to not-ignore: