MORE INFO:

Intensity refers to the weight used relative to your 1RM (1 rep max) in a given exercise. For example: Your 1RM squat is 275 pounds. If the program calls for 80% intensity, it means you’re doing 80% of 275 pounds. That’s 220 pounds. Simple. If it called for 100% intensity you’d be using your 1RM weight of 275.

Effort is how hard any physical work feels, which is independent of load. Consider these two scenarios:
Scenario 1: Heavy squats for sets of 3 reps at your 3RM. This will feel like a massive amount of effort, like you have to lift this weight to save your life and not get crushed under the bar.
Scenario 2: Much lighter squats for sets of 15 reps at roughly your 15RM. This will ALSO feel like a massive amount of effort. If you’ve ever done a set of squats for a 15RM you’ll recall lots of shaking, panting, and “just a couple more, come on, you got this” thoughts in your head.

The point being, both will feel like a lot of effort because you’re going to expend a lot of energy to complete the set in either case. The difference is that the sets of 3 are high intensity (about 90% of 1RM), while the sets of 15 are lower intensity by comparison (closer to 60% 1RM).

SOURCE:

https://www.t-nation.com/training/tip-intensity-vs-effort-the-real-story/