A restrained creature’s speed becomes 0, and it can’t benefit from any bonus to its speed.
Attack rolls against the creature have advantage, and the creature’s attack rolls have disadvantage.
The creature has disadvantage on Dexterity defence checks.
A creature that has been restrained via a grapple remains restrained as long as it is also grappled. If the grapple breaks, it is no longer restrained.
You can spend one of your attacks to make an unarmed attack roll vs the strength or dexterity defence (target’s choice) of a creature that you are restraining. If you succeed, the creatured is pinned by you. If you fail, the grapple is broken and the creature makes an opportunity attack against you.