You can withdraw into your shell as an action. Until you emerge, you gain a +4 bonus to your AC, and you have advantage on Strength and Constitution saving throws. While in your shell, you are prone, your speed is 0 and can't increase, you have disadvantage on Dexterity saving throws, you can't take reactions, and the only action you can take is a bonus action to emerge from your shell.
Shell Defense
Raw data
{
"srd": false,
"name": "Shell Defense",
"page": 34,
"type": "entries",
"__prop": "raceFeature",
"source": "MPMM",
"entries": [
"You can withdraw into your shell as an action. Until you emerge, you gain a +4 bonus to your AC, and you have advantage on Strength and Constitution saving throws. While in your shell, you are {@condition prone}, your speed is 0 and can't increase, you have disadvantage on Dexterity saving throws, you can't take reactions, and the only action you can take is a bonus action to emerge from your shell."
],
"raceName": "Tortle",
"basicRules": false,
"raceSource": "MPMM"
}