What you need is an ecu like Mo2 said. Basically, they reflash the maps controlling fuel/air mixture, when and how much fuel is delivered, and possibly advance/retard the timing.
You plug it into your OBD port and, depending on the unit, it'll either overwrite the current map, or some stay mounted in the truck, and 'bypass' it. The better ones will come with predetermined maps for your specific truck - stock, towing, performance, economy, etc.
For a stock vehicle, it really depends on how much power the manufacturer 'left on the table'. If your F150 in an eco-boost motor, there's a good bit of room to open up the turbo's, as Ford kept the tune relatively mild to meet fuel economy goals. If it's a Coyote block, I'm not sure what kind of gains are to be had.
I've been debating putting a tuner on my Ram 1500 to shift the curve for towing a bit, but I want to make it breath a little better first. That's where a tuner really comes in handy. Custom tunes help you take advantage of mods, and stock tunes can really hinder a heavily modded motor. My old 335i had a COBB unit installed to open up the turbos quite a bit (BMW left a lot of power to open up to begin with) and take advantage of the extra airflow from the upgraded intake/exhaust. My Kawasaki has a Dynotek installed with custom tuned maps for the work done to it, and it would run extremely lean without.
My recommendation is go find an F150 forum and start learning what others are doing, and what works/doesn't for your truck.