Melee Attacks

A melee attack allows you to attack a target within your reach. A melee attack typically uses a handheld weapon or an Unarmed Strike. Many monsters make melee attacks with claws, teeth, or other body parts. A few spells also involve melee attacks.
A creature has a 5-foot reach and can thus attack targets within 5 feet when making a melee attack. Certain creatures have melee attacks with a reach greater than 5 feet, as noted in their descriptions.

Instead of using a weapon to make a melee attack, you can use a punch, kick, head-butt, or similar forceful blow. In game terms, this is an Unarmed Strike-a melee attack that involves you using your body to damage, grapple, or shove a target within 5 feet of you.
Whenever you use your Unarmed Strike, choose one of the following options for its effect.
Damage. You make an attack roll against the target. Your bonus to the roll equals your Strength modifier plus your Proficiency Bonus. On a hit, the target takes Bludgeoning damage equal to 1 plus your Strength modifier.
Grapple. The target must succeed on a Strength or Dexterity saving throw (it chooses which), or it has the Grappled condition. The DC for the saving throw and any escape attempts equals 8 plus your Strength modifier and Proficiency Bonus. This grapple is possible only if the target is no more than one size larger than you and if you have a hand free to grab it. See also Grappling.
Shove. The target must succeed on a Strength or Dexterity saving throw (it chooses which), or you either push it 5 feet away or cause it to have the Prone condition. The DC for the saving throw equals 8 plus your Strength modifier and Proficiency Bonus. This shove is possible only if the target is no more than one size larger than you.

Opportunity Attacks

You can make an Opportunity Attack when a creature that you can see leaves your reach using its action, its Bonus Action, its Reaction, or one of its speeds. To make the Opportunity Attack, take a Reaction to make one melee attack with a weapon or an Unarmed Strike against the provoking creature. The attack occurs right before the creature leaves your reach. See also chapter Combat.

Grappling

A creature can grapple another creature. Characters typically grapple by using an Unarmed Strike. Many monsters have special attacks that allow them to quickly grapple prey. However a grapple is initiated, it follows these rules. See also Unarmed Strike and Grappled.
Grappled Condition. Successfully grappling a creature gives it the Grappled condition.
One Grapple per Hand. A creature must have a hand free to grapple another creature. Some stat blocks and game effects allow a creature to grapple using a tentacle, a maw, or another body part. Whatever part a grappler uses, it can grapple only one creature at a time with that part, and the grappler can't use that part to target another creature unless it ends the grapple.

Ending a Grapple. A Grappled creature can use its action to make a (Athletics) or (Acrobatics) 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. In addition, the grappler can release the target at any time (no action required).