Ghusl al-Janabah (wash of Janabah) is a special rite of washing to get the person out of janabah (an impure state). Janabah is a state which a person enters when they have had sex or ejaculated. They cannot pray if they have not done the gusl al-janabah (wash of janabah) in order to cleanse themselves.

The wash is very simple. A person makes the intention to wash, then they wash their head and neck fully, then they wash the whole of the right-hand side of the body, and then the left-hand side of the body.
Alternatively, they can make the intention and then plunge themselves into a vast (kurr) pool of clear water (not swimming pool water).
If there is no water or if there's no time to wash before the prayer time finishes, they must do tayammum temporarily.