Short answers, pulled from the story.
An experience point (XP or exp) is a unit of measurement used in tabletop and video role-playing games to quantify a player character's progression. Characters earn XP by completing objectives, overcoming enemies, and successful role-playing, and accumulating enough XP causes a character to level up and grow stronger.
Gary Gygax and Dave Arneson introduced experience points when creating Dungeons & Dragons. The abbreviation XP was suggested by Lawrence Schick, one of TSR's first hires, because EP was already in use for "electrum pieces" in D&D's currency system.
In RuneScape, no player can exceed level 120 in any skill, which requires exactly 104,273,167 experience points to reach. No single skill can accumulate more than 200 million experience points total.
The 1997 role-playing video game Fallout was the first to use the word "perks" for special bonuses unlocked through experience points. The term is short for "perquisite" and refers to permanent character bonuses that become available as XP accumulates.
Remorting is a mechanic where a character who has reached the level limit starts over from scratch in exchange for an advantage previously unavailable, such as access to new races, classes, or locked areas. The term originated in MUDs, where reaching maximum level made a character an immortal administrator, and choosing to return to regular play was called remorting.
Grinding is the practice of repeating the same activity over and over to accumulate experience points, sometimes called XP farming. Power-leveling is a related but distinct practice where a stronger player helps a weaker character gain levels much faster than would be possible alone.