Intensity in other words is the difficulty of an exercise, it can be measured in Kg’s, speed, Watts, or time taken. It can be adjusted with shortening or lengthening rest periods in between sets or exercises. This is all determined off of the context of your training plans goals.
When training, warming up or cooling down you will have prescribed intensity levels and these are based on the Rate of Perceived Exertion scale (RPE) or Alternatively the Reps In Reserve Scale (RIR). The RPE scale is simply a scale from 1 to 10 that ranks the effort from 1 being the easiest to 10 being the most difficult, the more training experience one has when using this scale the more accurate the ratings will be in relation to their true maximal efforts.