The term frigate has gone in and out of fashion in naval classification throughout human history, often popping back up when a midpoint between more established classes is needed.
In the 24th century, frigates are meant to be a midpoint between Rangers and Cruisers. Large enough to hold powerful weapon systems and supplies to stay on station for months, but not reaching the same capabilities of a cruiser in terms of crew capacity or a ranger's maneuverability.
The frigate makes for sending on defensive missions in poorly supplied areas or interplanetary missions that need to be done, but don't require the capabilities of a cruiser.
In the 24th century, frigates are meant to be a midpoint between Rangers and Cruisers. Large enough to hold powerful weapon systems and supplies to stay on station for months, but not reaching the same capabilities of a cruiser in terms of crew capacity or a ranger's maneuverability.
The frigate makes for sending on defensive missions in poorly supplied areas or interplanetary missions that need to be done, but don't require the capabilities of a cruiser.