Force fields enable you to surround yourself with an invisible shield for protection. They have often proven to be an invaluable defence system for superheroes, and can prevent all kinds of threats from harming the person with is power. In Lex Luthor's case, the force field he has is part of
one of his many war suits. This particular war suit was designed by the Apokolitians for him, meaning it uses all kinds of cool technology. The main purpose of his force field is to keep his head protected, allowing him to wear the suit without a helmet on.
The force field is strong enough to withstand minor energy blasts, such as Starfire's starbolts. Most force fields do have their limits though and Lex Luthor's is no different. While it can withstand a large amount of attack, if it's put under too much pressure it can collapse. This means he can't rely wholly on it for defense, and it leaves a bit of room for him to still be beaten in battle by heroes like Superman.