Yes coffins are sealed shut with a rubber seal that goes around the lid of the coffin.
When the coffin lid is closed on the deceased person the elements and hopefully insects, water etc are sealed out of the coffin to protect the body from much decay.
Although when the coffin is buried for 25 years or more the coffin and the seals can start to break down and especially in 1000 years or longer the coffin and body will be likely gone or at least the body will be nothing but bones.
Most coffins also just have latches that seal the lid down and they can be easily opened by lifting the latch.
In the old days before modern caskets the caskets were made of wood and nailed shut and then water, insects etc could all get into the coffin.
You can still bury someone in a wood casket if you want too but modern caskets help seal out the elements.