It's confuing to be sure, since the most common sources of training info don't seem to give any heed to the strength sports, who have standardized terms for this stuff. Anywho...
Intensity is the % of your most relevant 1 rep max that you are using. So if your 1rm on bench is 100 ppounds and you're using 85 pounds for 5 reps, you are using 85% intensity. So the suggestion to keep intensity high means to keep the weights high.
Volume is the total number of reps. Simple.
Frequency (which wasn't mentioned but is the third main facotr in programming) is how often one works out.
Workload is the amount of weight moved (weight x reps). Example: if you did 5 sets of 5 reps on bench with 85 pounds, your workload for that session is 2125 pounds (5x5x85). Assuming a 1rm of 100 pounds, you did a volume of 25 reps at 85% intensity for a total workload of 2125 pounds. You could track workload in a number of different ways - total workload in a week, workload in a training session, workload on a particular movement, etc. BTW you can track volume multiple ways as well.
Why do all this stiuff? To quantify progress. If you just go to the gym and work hard til you kinda feel like you've done enough, how do you know if your stimulus (training) is going anywhere? Conversely if over time you are able to up your workload through manipulating the variables, you are making progress. Tweaking the variables is where you gotta apply some art to the science and figure out what works for you. Your goals will affect the changes you'd make as well.
I guess that could be called "training theory 101"