The rank of First Lieutenant is an intermediate rank between Lieutenant and Lieutenant Commander. First Lieutenants may fill roles in Starfleet usually assigned to Lieutenant Commanders, and can hold positions like Chief Engineer and Second Officer, perhaps even filling the role of Acting Executive Officer.
This rank allows Captains to promote worthy officers to positions of greater authority and responsibility while allowing time for gaining experience, training, and approval from High Command for the next step up.
First Lieutenant in Star Trek Canon Edit
The rank of First Lieutenant is loosely based on a mix of United States Army and Navy traditional ranking systems.