It opens a fuzzy finder window showing the branches you have in the repo, sorted by recency. Also shows the latest committer and the date. Hit enter on the one you want to swap to it.
FYI/fwiw Fish shell autocompletes branches sorted by last commit date when writing out git commands. It also provides completions for commitish objects grouped by class, giving precedence to the usual targets first, with each group sorted by its own sort key. Handy!
I've seen monorepos with over 10k active branches. But even at a smaller scale (10-100 branches), it becomes useful when manually stacking and/or rebasing, especially if you have a smaller terminal.
[0] - https://github.com/paulirish/git-recent
It opens a fuzzy finder window showing the branches you have in the repo, sorted by recency. Also shows the latest committer and the date. Hit enter on the one you want to swap to it.