Mastery is a class feature shared between some martial classes that reflects their specialization with certain types of weapons, shields, and armor. Each piece of equipment lists a mastery property in addition to it's Properties. If the equipment meets the requirements in your class's Mastery feature, you can learn its mastery property and use it in combat.
Learning a mastery property isn't permanent. During an Extended Rest, you can practice weapon drills and swap out one of your choices at the end of the rest.
There are 16 total masteries, be sure to click Next Page!