PvE, or player vs entity, involves players, whether individually or in teams (generally with pvp turned off), attempting to survive multiple waves of enemies.
In most cases, you will be given some weapons, such as a stone or wooden sword, to begin with, along with armour such as chain. As you progress, often you will be given more powerful weapons and armour, along with potions of damage or poison. Remember to use a sword to block projectiles, and try to keep your back to an ally or a wall to prevent Flanking by mobs.
It is usually a good idea to remain relatively mobile. If you have a bow, try climbing a steep hill with only a few viable paths, to channel the mobs. If not, dart back and forth to attack, limiting the amount of time you are in weapons range of the monsters.