When you really need to poop the poop should normally easily come out without much need of pushing the poop out.
But sometimes when you need to poop it can be a struggle for the poop to come out easily such as when you're partially constipated or you have hard stools.
Pushing your poop out sometimes is not harmful as long as you're not pushing the poop too hard because if you do push the poop out too hard and struggle and strain to push the poop out then you could tear a tissue in your anus.
Sometimes in rare cases you can also rupture blood vessels in your brain and cause a brain aneurysm to occur.
So avoid struggling or straining to poop too much because of those potential problems.
If you're having to strain or struggle to poop then you're likely constipated or you have a bowel blockage.