Save a checkpoint of this computer — the state of the filesystem and everything installed, kept under a name so you can put the machine back to exactly this state later. The computer keeps running; nothing is interrupted.
Take one before anything you would struggle to undo: a database migration, a dependency or framework upgrade, a large refactor, a destructive script, or handing the machine to a different task. This is the persistent-computer version of "let me start fresh" — you cannot start fresh here, but you can come back.
Only the first checkpoint copies the whole computer; each one after it saves what changed since the last, so taking one before every risky step is cheap and is the intended habit. There is still a limit on how many are kept, so name them for what they are ("before the schema change") rather than by number.
The state is taken from the live filesystem, so finish or stop anything mid-write first — a database being written to, a build in progress — or that file is captured half-written.