Acceptance rate (or rejection rate) is the ratio of the number of articles submitted to the number of articles published. It can measure the selectivity or prestige of a journal, though like many journal metrics, the raw number is not the whole story.
There is no single list or database of acceptance rates. If the tools listed in the Journals Acceptance Rates Research Guide do not provide a rate for a journal, emailing the journal editor or other staff is likely the best way to get that information. UlrichsWeb - Ulrich's Periodical Directory will list contact information for a journal's editorial staff.
For more information, see the Journals Acceptance Rates Research Guide.