Smart Steering is a mechanic introduced in Mario Kart 8 Deluxe as a way to make gameplay easier.
With Smart Steering turned on, the player will auto-pilot along with the road's edges and not be able to go off-road. Also, an antenna is inserted onto the back of the player's Kart or Bike and will light up whenever the player moves along the edge.
Smart Steering is useful for those new to the Mario Kart series and those who tend to go off-road frequently (especially in the higher engine classes). However, it has disadvantages too. The Ultra Mini-Turbo cannot be accessed while it's enabled and off-road shortcuts can't be reached. Sometimes, Smart Steering may cause the chosen path to switch unexpectedly, which may cause frustration.