Player Versus Environment, or PVE, is a combat mode that allows players to fight monsters only, as opposed to PVP. Many different MMORPGs are PVE by default and servers labeled as normal servers are usually PVE. Although PVE servers have some PVP elements, those are usually restricted within the PVE environment.
The main focus of Tree of Savior are monster hunting and questing. IMC Games intends to balance quest-leveling and monster-leveling to be on par so that one is not more appealing than the other, giving players more flexibility in how they wish to level up.
Monster hunting is a core activity of MMORPGs and ToS is no different. Hunting monsters provide valuable EXP for leveling up in addition to item drops that players can use to either craft items or sell to non-player characters (NPCs) to earn money.
IMC Games originally had more quests in addition to automatic quest indicators in ToS, but eventually took out some of the more mundane quests and hid quest flags due to beta tester feedback. This gives ToS a much more role-playing game (RPG) feel to it since quests are fewer and hidden so players are encouraged to do more exploration to find quests as they would in a typical RPG.