Usually what it means is that the inside of the anus is exposed. That can cause a little pain and it can be exposed for prolonged sitting, especially on a western style seated toilet. However it can cause more pain if the water pressure of the bidet is too strong. In such a case the anus will usually fold itself back in just for walking and standing more. But there is no harm in pushing it gently back in manually, and this may even be necessary. Applying some mineral oil or olive oil will help with that process.
Constant exposure of the inside of the anus and the reasons that is happening (too much sitting mainly) are a source of hemorrhoids, which are bags of skin with bloated blood vessels in them which may protrude out the anus, be only in the anus or switch positions. Those will hurt intensely if hit directly with a stream of water. Also those hemorrhoids can tear causing more pain. And those tears form scars which easily tear again if feces is hard and large, and again cause intense pain when exposed and hit with a jet of water.
Anyone having such problems should lubricate with mineral oil or olive oil. They both also help if the anus has become dry or chafed for any reason, and both of those can also lead to tearing.
The anus is a sensitive part of the body which people abuse in modern times especially though excessive sitting. The anus did not evolve for sitting. Anyone who sits a lot should learn to clench their butt cheeks around the anus when sitting down, rather than allow the butt cheeks to spread and the anus to get squashed between themselves and a chair, or even be spread out for long periods. And for those who ignore this advice with a snicker, enjoy your middle age hemorrhoids. You have been warned.