Varia Suit is the first suit upgrade in Metroid Prime. It is one of the items that Samus loses in the Space Pirate Frigate, consequently downgrading Samus' suit to the Power Suit. It is guarded by Flaahgra in the room Sunchamber.
The main use of Varia Suit is to avoid taking heat damage in the Magmoor Caverns. Most of the rooms in Magmoor Caverns have a flag set which detects if Samus is wearing the Varia Suit, and she will take damage at a constant rate if she enters these rooms without Varia. If Samus doesn't have Varia, but has a later suit upgrade such as Gravity Suit or Phazon Suit, this effect still applies in the vanilla game. Randomizers may make it so that Gravity and Phazon suits do not apply this effect to Samus.
Varia Suit can be skipped in 20%, but the method of doing so is currently TAS-only, meaning that this doesn't apply to human Low%, which is 21%. The only other method of skipping it is by grabbing up to five energy tanks and performing what is known as the Varialess Magmoor Run, known as such since it's a route that goes through Magmoor without getting the Varia Suit.
Additionally, Varia Suit applies a 10% damage reduction to Samus.