Weapon Mastery
Your training with weapons allows you to use the mastery properties of three kinds of Simple or Martial weapons of your choice. Whenever you finish a Long Rest, you can change the kinds of weapons you chose. For example, you could switch from using the mastery properties of the Longsword and Shortsword to the mastery properties of the Halberd and Warhammer.
When you reach certain Fighter levels, you gain the ability to use the mastery properties of more kinds of weapons, as shown in the Weapon Mastery column of the Fighter Features table. At 4th level you can use three weapon masteries, at 10th level you can use four, and at 16th level you can use five.