Means a massive amount of something. It’s one of those properly British made-up measurements where nobody’s done the maths, but you still know it’s loads. A shedload can be food, work, emails, drama, whatever. It’s a bit daft, very useful, and it sounds funnier than saying a lot.
"I said bring snacks, not a shedload of sausage rolls. Now Dave’s turned up with a shedload of mates and the kettle’s already on its last legs"