In order to run and start the pistons must be able to compress the air drawn into the cylinder sufficiently to get the correct air to fuel ratio needed to start and run. If an engine has extremely low compression you will hear the starter motor spin it very quickly or you will feel little resistance on the pull chord.

The head gasket on the engine is blown

There is a gasket that seals the cylinder head to the engine block to prevent the combustion gasses escaping and to keep the cooling water (and oil on a four stroke) out of the cylinders.

Worn or broken piston rings

Piston compression rings primarily function to seal the cylinder walls, containing the air fuel mixture so it can be compressed as the piston rises. If the compression rings are worn or broken, compression will be low.

There is a hole in the top of the piston

A hole in the top of the piston will cause the cylinder to have essentially zero compression.