In a nutshell, it's a condition that comes from a a prior job in the schedule that will not always run.
You have a job, Z, which backs up a database. You want to run it only after all of the jobs that update said database have finished.
Your Monday-Thursday schedule has three jobs, A, B, and C which update that database. On Friday and only Friday, there is a fourth job, D.
So you set up job Z to have jobs A, B, and C as in-conditions, and to have job D as a Maybe Condition. Control-M will only wait for D if it has been scheduled, otherwise, it will only wait for A, B, and C.
There are a lot of little rules for this, for things like when did job D get placed in the AJF, and if job D is in the right "groups." So be sure to study it all as if you are taking a test. You can also use those built-in utilities to test your schedules without running them.