There's this common scenario that you have a PR in review, the reviewer is requesting some changes, and you make those changes and would normally simply squash them into the original commit that they came from. If you do that, however, there's no way for the reviewer to see what you changed. You could just make a separate commit with those changes at the end of the branch, but this is not ideal because it results in a git history that is not very clean.
To help with this, git has a concept of fixup commits: you do make a separate commit, but the subject of this commit is the string "fixup! " followed by the original commit subject. This both tells the reviewer what's going on (you are making a change that you later will squash into the designated commit), and it provides an easy way to actually perform this squash operation when you are ready to do that (before merging).
You could of course create fixup commits manually by typing in the commit message with the prefix yourself. But lazygit has an easier way to do that: in the Commits view, select the commit that you want to create a fixup for, and press shift-F (for "Create fixup commit for this commit"). This automatically creates a commit with the appropriate subject line.
Don't confuse this with the lowercase "f" command ("Fixup commit"); that one squashes the selected commit into its parent, this is not what we want here.
There's a special type of fixup commit that uses "amend!" instead of "fixup!" in the commit message subject; in addition to fixing up the original commit with changes it allows you to also (or only) change the commit message of the original commit. The menu that appears when pressing shift-F has options for both of these; they bring up a commit message panel similar to when you reword a commit, but then create the "amend!" commit containing the new message. Note that in that panel you only type the new message as you want it to be eventually; lazygit then takes care of formatting the "amend!" commit appropriately for you (with the subject of your new message moving into the body of the "amend!" commit).
When you're ready to merge the branch and want to squash all these fixup commits that you created, that's very easy to do: select the first commit of your branch and hit shift-S (for "Squash all 'fixup!' commits above selected commit (autosquash)"). Boom, done.
When you are making changes to code that you changed earlier in a long branch, it can be tedious to find the commit to squash it into. Lazygit has a command to help you with this, too: in the Files view, press ctrl-f to select the right base commit in the Commits view automatically. From there, you can either press shift-F to create a fixup commit for it, or shift-A to amend your changes into the commit if you haven't published your branch yet.
If you have many modifications in your working copy, it is a good idea to stage related changes that are meant to go into the same fixup commit; if no changes are staged, ctrl-f works on all unstaged modifications, and then it might show an error if it finds multiple different base commits. If you are interested in what the command does to do its magic, and how you can help it work better, you may want to read the design document that describes this.