What does districts must be compact contiguous and substantially equal in population mean?

Answer 1

It means that Gerrymandering is less easy.
Compact: Think of dropping a big spoonful of honey on a table. The result will be a round blob. We would say that the area of contact between the honey and the table is compact. In contrast, if you were to drop a spoonful of water from a couple of inches above the table it would splash out. There would be 'arms' of water that would spread out from the central wet area on the table. The area of contact between the water and the table would not be considered compact.

Contiguous: The city of New York is not contiguous. There's the mainland and there's Long Island, and these pieces are separated by salt water. In contrast the state of Idaho is contiguous--because it's all one big area.

Equal in population: obvious.