If something's unbolted, it's not locked or latched. Don't leave the back gate unbolted when you leave, or the goats will get out!
When something is unbolted, its lock or bolt hasn't been secured in place. You can also say it's unlocked or unlatched. An unbolted front door can be opened by turning the knob, and an unbolted bike lock is basically useless against bicycle thieves. Unbolted comes from bolt, which as a noun has meant "stout pin for fastening objects together" since the 15th century.