Imagine that you are doing a commit and you forgot to bring a file or multiples files. You can do a second commit but this would have the consequence to have two commits for something that in fact would have been in one. Git allows you to alter a past commit with the amend parameter.
For example, here is a small commit of 1 file.
git add main.cs git commit -m "This is my first commit"
This is how to add a second file to this first commit.
git add util.cs git commit --amend --no-edit
The –no-edit parameter specify that we do not want to modify the message. If you are doing a git log, you will see a single commit.