The obvious answer is the migration is not supposed to be stopped, but you might disable the network or otherwise halt the machine that is sending the data.
It should be fine since it's just sending data.
The receiving mac might handle the interruption well or not. Something was working on the mac to start the migration assistant - so you should be able to get back to that state fairly easily.
There are specific steps to clean up based on what part of the transfer was in progress.
Safest is to go back to a sane backup and attempt migration again with a faster connection or more time.
Post what happens here or as a follow on question - it's pretty easy to clean up the user accounts if they are the part that got interrupted (instead of apps, system settings, or random non-user files being transferred)
Half a user is usually what you end up with and that's not good in general.