git add . adds / stages all of the files in the current directory. This is for convenience, and can still be used if you have certain files you don’t want to add by using a .gitignore
A tutorial for .gitignore is located here.
A deeper look into git add . vs git add -A vs. git add -u is located here and it might answer your question if you wanted more control of how you add all of the files / wanted to know how git add . works.