If you hold in your poop for too long you can become constipated and will have a hard time pooping later.
The poop being backed up in your body can also cause problems with your bowels and can cause deadly bacteria to build up if you hold the poop for too long.
When you have to go poop you should always get to a toilet and relieve yourself as soon as possible.
Holding poop for too long can be deadly as well.