A creature can grapple another creature. Characters typically grapple by using an Unarmed Strick. Many monsters have special attacks that allow them to quicly grapple prey. However a grapple is initiated, it follows the rules of Condition Grappled and Unarmed Strike.

One Grapple per Hand: A creature must have a free hand (or other part like tentacle, maw, etc.) to grapple another creature. Once grappling it cannot use that part to grapple another creature or attack until ending the condition.

Escaping a Grapple: A Grappled creature can use it's action to make a STR (Athletics) or DEX (Acrobatics) check against the grapple's escape DC, ending the condition on itself on a success. The condition also ends if the grappler has the Incapacitated condition or if the distance between the grappled target and the grappler exceeds the grapple's range.