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. The method of calculating acceptance rates varies among journals. Some use all manuscripts received as a base for computing this rate, others allow the editor to choose which papers are sent to reviewers and calculate the acceptance rate on those that are reviewed that is less than the total manuscripts received. Also, many editors do not maintain accurate records on this data and provide only a rough estimate. Furthermore, the specialization of the field influences the acceptance rate - if only a few people write papers in an area it tends to increase the journal's acceptance rate.
There is no single list or database of acceptance rates. If the tools below don't provide a rate for a journal, try emailing the journal editor, or checking the journal website (you might want to google the journal name and acceptance rate).