When to UNDO/REDO?

You would typically want to UNDO/REDO when you commit some changes to git, and realize that the changes need to be removed/reverted. This very common in scenarios for example, when you did a temporary change to some files, and forgot to revert them, then proceeded to adding them to commit accidentally.

The UNDO/REDO workflow:

Assuming you did some changes and made commits like:
git commit -m "Commit 1 - Some changes to the code"
git commit -m "Commit 2 - Some MORE changes to the code"

Step 1 (UNDO-ing): Revert back the last commit
git reset --soft HEAD~
Step 2: Do the changes.
Step 3: Add your files to the staging area
git add <filenames or paths> or git add --all
Step 4 (REDO-ing): Do the commit.
git commit -c ORIG_HEAD or git commit -C ORIG_HEAD

How does this work?

Now that you know the flow lets understand how this works behind the scenes.

1. Step 1 resets the last commit i.e. "Commit 2 - Some MORE..." back to the "Commit 1 - Some..." commit.
2. In Step 2, you do changes you deem fit to the files.
3. In Step 3, you add the changed files to the staging area either selectively with git add <filenames> or all files with git add --all.
4. In the final step you commit the changes in the staging area.

Note: you can either use -c or -C. The small -c will open an editor for modifying the commit message, in this case it will be Commit 2 - Some MORE.... You can edit the commit message as you want.

Or alternatively you can use caps -C, where git will skip the editor window, and reuse the LAST commit message which again in this case is Commit 2 - Some MORE....

Re-using the "Same" commit message is also known as redoing/recommiting.

Some More tricks:

You can go back any number of commits by using git reset --soft HEAD~n where you want to undo last n commits.

Attribution: This article is based on a Stack Overflow question here.