The insurance protects you against hitting the bottom multiplier as you will receive your buy-in back instantly if you hit the x2 Multiplier. The insurance is to bet for the "x2 Multiplier". If the "x2 Multiplier" is selected, you will receive the insurance payout and buy-in back. You will essentially win a free game by getting your buy-in returned immediately upon hitting the 2x buy-in.
The insurance fee is the same amount as the buy-in. If you Spin any other multiplier, no insurance payout will be given and the game will continue as normal.