Most character abilities can be used once and then they're disabled. Use a defend command like Rest to recharge your character abilities.
Besides their own abilities, characters can also use items and programs. Items and programs recharge at the start of battle, but can't be recharged mid-battle. Items are shared by the party. Search for item treasure chests carefully to gain new items. Programs are attached to shields. Equip the right shield and if your hackitude stat is high enough, you'll be able to use its programs.
In the main menu, you can equip abilities, items, and shields. You can also read detailed information about different abilities, programs, and items.