Capitán is guerrilla rank 8 in Libertad. It unlocks a late-game batch of weapons and gear from Juan and his Arms Dealers.
Your guerrilla rank rises as you earn XP across Yara. Each new rank opens a new batch of weapons and gear for purchase, and some items say right in their unlock text which rank you need. The game shows either the feminine or masculine form of each title.
By now most of Yara is open. Clearing remaining bases and Yaran Stories will carry you to Mayor.