When you use plastic at some retailers (gas stations in particular), they 'block' your card. This means they authorize and reserve money in your account - typically $50 to $100. They don't know exactly how much you'll buy, but they want to make sure you can afford it. If you only buy $20 worth of gas, they won't release the remaining amount immediately -- it can take several days. During that time, you don't get to use the blocked money and you might bounce checks or incur overdraft charges.
There is no difference in how retailers block debit vs credit cards.