The Intensity metric is SPT's secret algorithm. The number tells you how hard you've worked, and how intense the performance was. You can use this number to compare yourself against your teammates and your older performances.
A very high level view of it is that it uses your speed at points throughout the performance to understand when you were performing high velocity movement, and identified if you've been doing that for a while, and how many of them you've done throughout the performance.
A really intense and brutal session can sometimes yield a score of above 100. The fittest and fastest players in elite squads would get somewhere in the range of 105-125.