Volume is simply the amount of weight times the amount of reps, times the amount of sets you do of a particular exercise. So if you Squat 200lbs for 8 reps for 3 sets, you wind up with 200x8x3=4800lbs total moved weight. This is volume.
Frequency is how often you work a particular muscle group or a particular lift. So if you lowered your Squat days from 3 times a week to 2 times a week, you would be lowering the frequency. If you increased your Squat days from 3 times a week to 4 times a week, you would be increasing your frequency.
Intensity is a way of talking about the difficulty of a lift with regards to your 1 rep max. So if you squat 300lbs for your 1 rep max, then a low intensity of 50% would mean doing reps with 150lbs, but a high intensity of 90% would mean doing reps with 270lbs. Generally, the higher intensity the lower the reps, and the lower intensity the higher the reps.