Spot instances are like not used instances that you can bid for. So, if your bid price is greater than that of spot instance price, a spot instance is launched. But the problem is if anytime spot instance price becomes greater than bid price, the instance can go away.
But you don't have that problem with on-demand instances because. On-demand instances are available to you whenever you need them and you need to pay for the time you use them on an hourly basis.