What is the meaning of 'Tilting' or 'Being on Tilt'?

Tilt originated from Poker and it's usually a state of emotional frustration and confusion.

It's most commonly used if you're going on a losing streak and then you become so frustrated that you start playing worse because you cannot focus anymore.

The Dignitas Blog wrote a nice article about tilting and how to prevent it.

"Tilt" originates from pinball tables, where altering the slope of the table would affect the path of the ball. As this is essentially cheating, pinball tables were designed with a tilt sensor, which when triggered would disable all scoring mechanisms (and usually the flippers as well), forfeiting any further action with that ball.

A player that's frustrated may shove the table in anger, triggering the tilt sensor. (Gentle nudging was a common tactic, if one was familiar enough with a particular pinball table to know the sensitivity of the sensor.) So the expression "on tilt" started to be used to mean a player who's lost his cool.

"On tilt" is a term used to describe a negative emotional state that clouds a person's judgement and leads them to continue to make mistakes, often growing in magnitude as the player's frustration mounts.

The term is often used in reference to gambling, as well as competitive games many sorts, including video games and poker. Wikipedia has an article dedicated to the use of the term in the context of competitive poker play.