In British English, it means a very thick slice of bread. Cafés sometimes offer 'doorstep sandwiches' made with two very thick slices of bread. It doesn't mean the bread is particularly heavy, its just a reference to the thickness of the slices.

I agree with Andrew. I think you're in the UK, aren't you Jessicamessica? A doorstop is a really thick, chunky slice of bread

If the yeast in your bread isn't good, the bread won't rise the way it should. It will be very hard. People call a loaf of bread like this a "doorstop." Why? After baking, it's so hard, you can prop a door open with it. It will support the weight of the door because it's so hard. It could be a "doorstop" (an object that keep a door open so it doesn't close).