it refers to accepting their offer (on their applicant portal).
Now with the post-April 30th deadline, it also mean you must either "plan to enroll" OR "commit to enroll" on AMCAS.
"plan to enroll" allows you to hold a space at UC Davis while you might be waiting for another school from whom you have not yet received an acceptance. You must withdraw all other A's with this option.
With "commit to enroll" you are 100% certain you want to go to that school and must withdraw all applications from every other school.