This used to be true. As of 2017 or 2018, it is no longer a bidding system, but a published price that AWS offers based on capacity.
More details here:
https://aws.amazon.com/blogs/compute/new-amazon-ec2-spot-pri...
More details here:
https://aws.amazon.com/blogs/compute/new-amazon-ec2-spot-pri...
You set your max price, up to the on-demand price, and then you'll get your VM... or not. If you do get the VM, you still don't pay the full price in most circumstances unless demand is very high.