Intensity is the amount of weight or resistance lifted per repetition. The intensity that you use can be calculated as a percentage of your 1 rep maximum. For example, if your 1 rep maximum (the most you can lift in 1 single repetition) on the squat is 100 pounds, and you are working out with 75 pounds, the intensity of your lift is 75% of your max. Different goals require specific intensity levels for training:
Endurance requires 40-60% of your 1 rep max
Hypertrophy requires 60-80% of your 1 rep max
Strength and Power requires above 80% of your 1 rep max