Muscle fatigue after weight lifting?

It means that your muscles have been 'challenged' by the activity, and are exhausted.

This will stimulate them to reinforce themselves while they 'recover' from their exhaustion.

If you let them complete this recovery process, they will be stronger, or bigger, or more powerful, depending on HOW you exhausted them.

But if you train again too soon, this develpment will be interrupted, and they will take longer to inprove their performance.

Entirely normal. The fast-twitch muscle fibers you use during lifting use chemicals manufactured by your body....ATP and Creatine Phosphate. A heavy lifting session will deplete these chemicals and you will feel fatigued until your body replenishes them.

This is why an after-workout recovery drink or meal is very popular.