What are Supersets / trisets / giant sets?

Supersets are doing two exercises back to back with no break. 
Trisets are doing three exercises back to back with no break. 
Giant sets are doing 4 or more exercises back to back with no break.

During these sets you can either pair exercises that are non competing , in other words opposing muscle groups, or you can target the same muscle.

What is the point in this?
It increases the intensity and the time efficiency of your training. So if you want a more elevated heart rate, or your short for time or you want a more intense workout these could be ideal for you to include in your training. Another benefit of paring exercises is the increased density. This results in a level of fatigue and metabolic stress, which can create a more favorable hormonal response. As a result this may lead to greater muscle growth.

You can use your multiple sets to either pre exhaust or post exhaust a muscle before or after a compound lift. For example you could do leg extension before squats. Or do jumping lunges after weighted lunges. You could also do multiple exercises on the same muscle to burn it out. Or if you wanted you could do a full body circuit utilizing a giant set.

If your goal is to get stronger I wouldn’t recommend too many Supersets / trisets / giant sets ect as there will be an increased level of fatigue from complying several exercise which will limit your ability to generate your maximum force. Even if you are supersetting opposite muscle groups, they accumulated work will fatigue your muscles a lot more quickly thus decreasing your ability to produce force.

Now if your goal is to get stronger/gain muscle and loose fat I would recommend starting your training session off with classic body building and focusing on compounds lifts and then ending it with Supersets / trisets / giant accessory sets.

All training styles have their place in a program depending on the stages you are at in you fitness journey.