jj describe gives a name to a commit. In jj, everything rewrites the history, so there's no real point in calling it out in the command name since it's just the default behavior.

describe is also the command you can use to edit the commit message of the change you're currently drafting. In jj there's no staging area, every modification to the working tree immediately gets integrated into the current commit. (This means if you have no diff in your working tree, you're actually on an empty commit.)

in jj the history has mutable and immutable commits.

Not really familiar too, but jj has everything committed by default (no index, staging area, and uncommitted changes). You use ‘jj new’ to stop adding changes to the current commit.

‘jj describe’ lets you add a message to a commit as it’s not there by default.