Like other Battlefield games, your character levels up with experience. If you want to level up at a faster rate, use XP Boosts. These special items are found in Battle Packs. To use an XP Boost, you need to open up the pause menu with the Start or Options button. From here, you can activate the boost. Boosts can give you 25% to 200% extra XP, so they are well worth it, especially in modes like Conquest or Rush.