Committing means pledging your support for a sequel before it reaches its goal. You won't be charged until the book hits its commitment threshold. If it doesn't, you won't be charged at all.

Pre-ordering means the book has already reached its goal and is a confirmed Allurial Series pick. When you pre-order, you’re immediately charged $16.99.

Both committing and pre-ordering allow you to get the book at a discounted price, it's just a matter of timing.