It doen't always happen to every firearm, but loose breech is a serious issue that, left unchecked, poses a significant risk to the useful life of a firearm and the safety of the person firing it.
Before a loaded rifle or pistol is fired, the breech (that part of the receiver that pushes a round into the chamber and extracts the empty case from the chamber after firing) is supposed to seat securely against the opening of the chamber with no movement forward or backward. With time, especially in semi automatic rifles and pistols, the metal can become misshapened to a point where when the bolt is manipulated with fingers, noticeable movement back and forth can be observed...that movement is called "Loose Breech."
Left unchecked, the metal at the breech face and the chamber opening will continue to loosen the safe closure of the breech and could damage the firearm to a point where significant repair of metal surfaces or replacement of the bolt could become necessary. Loose breech can also compromise the breech's closure and locking system to a point where gas and fragments could escape and injure the firer.
If you know or are unsure if your firearm has loose breech, take it to a qualified gunsmith. The sooner loose breech is identified, the less it will cost to repair.