For resistance training, workload is the primary measure of intensity. Workload can have three components:
1. The amount of weight lifted during an exercise
2. The number of repetitions completed for a particular exercise
3. The length of time to complete all exercises in a set or total training session
So, you can increase workload by lifting heavier weights. Or you could increase the number of repetitions with the same weight. Finally, you could lift the same weight for the same number of repetitions but decrease the rest time between sets.
However, only increase the intnesity using one of the above parameters. Do not increase weight and decrease rest time in the same session for example.