Mayor is guerrilla rank 9 in Libertad and the top of the numbered ranks. It unlocks the last rank-gated 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.
Pick up any rank 9 items from Juan, then keep earning XP if you want to chase Comandante and the Glorious Leader achievement.