From the following excerpts:
Monk class Feature - Martial Arts
You gain the following benefits while you are unarmed or wielding only monk weapons and you aren’t wearing armor or wielding a shield:
• You can use Dexterity instead of Strength for the attack and damage rolls of your unarmed strikes and monk weapons.
When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you’re able to make multiple attacks with the Attack action, this attack replaces one o f them. The target o f your grapple must be no more than one size larger than you, and it must be within your reach.
Using at least one free hand, you try to seize the target by making a grapple check, a Strength (Athletics) check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). If you succeed, you subject the target to the grappled condition (see appendix A). The condition specifies the things that end it, and you can release the target whenever you like (no action required).
Escaping a Grapple. A grappled creature can use its action to escape. To do so, it must succeed on a Strength (Athletics) or Dexterity (Acrobatics) check contested by your Strength (Athletics) check.