I was put on Probation with a stay of adjudication and a 18 Day Cap, What does a 18 day cap mean?

It means the judge cannot sentence you to more than 18 days in jail, and can also order less or none if the judge chooses. It is a maximum possible sentence. The judge could usually also be free to choose instead to order work release, community work service through county work crews, or electronic home monitoring.

It sounds like you have plead guilty, but haven't been sentenced yet? If so, then the cap refers to maximum number of days in jail that the judge can give you at sentencing.

Generally, a 'cap' means an upper limit. You ought to discuss such matters with your attorney. You may read State V Moody, 2011, MN App., for some elucidation on the concept if you wish. Tricia Dwyer Esq.