An Optional PvP world (or PvE (= player versus environment)) is a world without PvP enabled. Secura was the very first Optional PvP world and is one of the busiest servers of Tibia. Optional PvP worlds have some limitations compared to regular worlds:
It is not possible to attack players.
Summons are essentially considered players, so they may also not be attacked and may not attack other players or summons. This also means that you cannot train on players or summons.
Skulls do not exist at all, not even the green skull.
It is not possible to heal wild creatures.
Attack runes used on other players/summons will have no effect.
Spells that create a damaging field can be stepped on by players without consequence (generates a "puff" graphic). They will also be a different color compared to normal fields so you can tell the difference between a player/summon-made field and a normal field.
Although player killing is not possible, be cautious where you are idle, since you can be pushed to creatures that will cause your death. Even rats from Thais sewers are quite commonly used to kill AFKers especially in Secura. Other common ways to do this is i.e. pushing to slimes roped from Carlin sewers. This tactic can also be used to kill botters.
Also because player killing is not possible and because the servers are so busy, scammers, kill/loot(bag) stealers, other thieves and otherwise annoying people are a not-too-uncommon sight.
An Optional PvP world usually has a name that in some way is connected with peace. Examples are Secura, the secure world; Calmera, the calm world; Honera, the honest world; Celesta, the celestial world and Guardia, the guarded world.