...
Once you have made changes to a file and want to commit these changes, you'll first "stage" the commits. The line command for this is git add <filename>, but github desktop allows you to click on a box to identify files you want to add to a commit and then you add a comment and add and commit by clicking on "Commit to master". If you make changes to a file that you don't want to keep, you can right click on the file name and select "discard changes." That will pull the old version of the file from the (invisible to you) local repository into your working directory. The command-line equivalent of discarding changes is git checkout.
After you have committed your changes to the local repository, you still need to push those changes to github for them to be in the remote repository.
...