If you hold in your farts too much you can develop fart breath.
The gas in your intestines will build up and reabsorb into your bloodstream. The gas then gets carried throughout your body and into your lungs which then comes out when you exhale.
So you'll be farting out your mouth instead of your anus.
Holding in farts can also become dangerous in certain situations. The dangerous situations that the farts will cause pouches to form in the wall of the colon. This condition is known as diverticula.
It's best to just fart and not hold it in. Everyone farts and it's a natural thing to do.