Experience is an addon that lets you get a snapshot of another player's dungeon experience at a glance. You will set a "fully experienced" threshold (5 by default), and a player is considered to be fully experienced if they have killed all the bosses in that instance at least that number of times.
Experience will not tell you how skilled a player is, or if they choose the right gems for their gear or whatnot, but it will tell you how much experience they have in various dungeons, letting you make decisions about whether or not to slot them for a run at a glance. Don't take it as an end-all be-all rating of a player (after all, the player behind an alt might have experience that the character does not), but you can use it to eyeball someone.
By default, the info window will pop up when you target someone. It will never pop up in combat, and can be toggled on and off with a handy LDB entry.