Circuit Training is a method of resistance training or weight training maximizing the volume of work done in a limited period of time.
It consists of performing several exercises on multiple body parts in a row, with little rest in between exertions. The aim is to train different muscular groups consecutively.
The most common sequence is legs exercise then arms exercise then core stab.
With this sequence your legs have the time to recover between two reps as even if you will continue to work you will mobilize other muscular groups.
Important point will be to not allow a too long recovery between two exercises as the aim is to stay is the burning fat training zones.
Here is an example of the Circuit Training I used to do on weekly basis when I start after my off season :