During exercise, to use your muscles beyond their limits. While being effective to build muscle, doing this at such an early time (Ex: Starting to exercise entirely) it can cause joint or muscle damage and is not advised. Try toning muscle at light weights for a couple months before trying this.
Failing Out: Early= Hurt Muscles and Joints
Later= Effective for Muscle Building
"After Failing out at the Gym Yesterday my arms are looking even better!"
"Ouch, my arms are killing me after I tried failing out last night."