A weapon is a piece of equipment that a character can use to attack another character, usually physically.
Depending on the role-playing game, the weapon a character uses might be an aesthetic choice (potentially affecting fictional positioning but not the game mechanics) or it might have traits and stats that influence how good the character is at attacking. Weapons can affect an attack's execution (how likely an attack is to hit, e.g. by determining a to-hit modifier) or its effect (how much impact the attack has, e.g. by determining how damage is calculated).
In games where weapons have mechanical weight, obtaining better weapons is a common means of character advancement. This is particularly common in traditional fantasy role-playing games, in which characters might be expected to acquire more powerful magic items as they level up.
Swords are a very common type of weapon, most notably in fantasy games but also in other genres.