After entering a Prestige, you will reset your championship progress (first "Regular" garage) and start over. Ultimate parts and currency from the second garage will not be removed. Once you enter a Prestige, you won't be able to undo the action. The support team won't be able to undo your progress either.
To be precise, here's a list of changes if you Prestige. 
You will get the following:
  • A special icon next to the character name that shows your Prestige Rank
  • Unique sticker
  • 30 Gems
  • 3 Skill Points
  • 100 Ultimate cash per stage and 200 Ultimate cash per crown (instant win, marked as a crown on the championship map)
  • Access to the new parts (chassis and weapons)
You will keep the following:
  • Ultimate Parts and Ultimate Currency (everything from the ultimate garage)
  • Gems
  • Co-pilots and their resources
  • Skills and Skill Points
  • Outfits
  • Monkey wrenches
You will lose the following:
  • All coins (gold)
  • All stickers
  • Everything from the championship garage (Regular First Garage)