A Jiu Jitsu Gi is the training uniform, similar to a Judo gi except that the cloth tends to be heavier.
Some of the advantages of training with a gi are that it is hotter! When you train with a gi, you sweat more and it increases resistance.
So when you train without a gi, you feel light and fast.
It is debated whether training gi or no-gi is better for fighting.
Some people argue that training with gi is better and helps you to focus on the leverages more.
Others feel that since your opponent in an mma match or street fight won’t be wearing a gi, that it is better to train no-gi.