If you've been lifting for a while then by now hopefully you all know that lifting heavy is key to gaining muscle. However, when we started and a lot of people now misunderstand what is actually meant by having to lift heavy. Bodybuilding is not about becoming a weightlifter, it's more about using the weights as a tool to build muscle. So throwing as much weights on the bar and only being able to do half reps or doing bicep curls with such a heavy weight that you have to throw your lower back into it is just called bad lifting, and rather than getting stronger or building muscle you're most likely just injuring yourself.
When lifting heavy to gain muscle you should find a weight that is going to be challenging for 8-12 reps but your able to
1) maintain strict form throughout the motion.
2) have a mind to muscle connection, focusing on mainly targeting the muscle you are working on
3) increase time under tension by slowing down the eccentric portion of the movement.