Electrolytic capacitors are usually DC only, but they can be designed to be used in AC power applications. The AC marking emphasizes that it is OK to use the capacitor in an AC application. You would want a capacitor clearly rated for the AC voltage in that application.
Do capacitors block AC?
More people should wonder why it does pass AC, rather than wonder why it blocks DC! After all, a simple capacitor, represented in diagrams, has two metallic plates in parallel separated by a small distance by air, a dielectric. By default, current can’t cross the dielectric, so that’s simply why DC can’t pass.