High reps with lower weight can lead to muscle fatigue; moving the muscle so many times, even lightly, can tire it out. The 'burn' you feel is probably lactic acid; muscle fatigue is when you can't move the muscle for one more rep.
Not sure of what results you are looking for, but high reps/low weights will not build muscle. It can lead to muscle toning, but that is more a function of losing fat, not building muscle. To build muscle and really, really feel the fatigue, you must lift heavy with fewer reps.
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