I don't understand when you use the -a flag while committing to Git? Can someone please explain?

Sure no worries. Let's say you have one file that you told git about by doing the following.

git add file.txt

And then moments later you make a change to that file in your text editor...Now if you were to do

git commit

by itself, without the -a, in order for your changes to actual be committed to git, you would first have to re-add the file, and THEN commit it, like so...

git add file.txt
git commit

another change 10 minutes later,

git add file.txt
git commit

and then another change 10 minutes later,

git add file.txt
git commit

And so you would have to do this re-adding process every time you want to commit changes to file.txt...Now an easier route is, since git starts tracking your files after the initial add command, instead of re-adding the file every time you want to commit changes, you just throw the -a flag after the commit command. Like so...

git commit -a file.txt

another change 10 minutes later,

git commit -a file.txt

And so it shortens the process of having to type things out.

The -a flag for commiting just tells git that you want all your files commited. It would be the same as giving it the --all flag, however, it will not commit any files that you havent told git about. Meaning that if you had just created/moved a file into the git directory that you're working on, but didn't do a git add command for that file, when you commit your changes, git will not include that file.