It varies according to the contract/agreement existing at the airline. Let me give some examples.
Block guarantee might be a minimum to be paid for the month. If you are on reserve, for instance and fly only 20 hours in the month, you might have a block guarantee that pays a minimum of 75 hours. If you fly 85, then you would get paid 85.
A Duty Rig guarantees a minimum pay depending on how long the duty day is. For instance, if you had a 1:2 duty rig then you would be guaranteed at least 1 hour pay for every two hours on duty for the day. That is, a day that was scheduled for 4 hours of flight time in a 12 hour day would pay at least 6 hours.
A trip rig would pay a minimum amount of flight time per hours away from base. For instance, if you had a 4:1 trip rig and you were away from your domicile for 80 hours, you would get paid no less than 20 hours for that trip.
Some airlines pay per flight hour + a per diem for each hour you are away from your base. Any guarantees from minimum daily, duty rig, trip rigs, cancelled flight, minimum monthly block, historic block to block would be factored in as well. Some airlines pay the scheduled amount if the flight is cancelled and some don't.