reserve guarantee is exactly that, if you are on reserve you are guaranteed to get 75 hours of pay at whatever seat/year rate that you are even if you never get called and do nothing the entire month. Basically you can count on that pay. If you fly some number under 75 hours you will still get 75 hrs pay.

Now, say the wheels are falling off (happens a lot here) and you are used every day you are on and
fly 83 for example. You will be paid above guarantee for those 83 hours because you exceeded reserve guarantee. Therefore your check(s) will reflect 83 hrs of pay.

Note that per diem applies too so when you get called out to fly, you will get per diem 24/7 from show time at base for that trip till duty off on the last day.