"Scream" can be either a verb or a noun, either describing the action or the sound made when one discovers that one's mother-in-law is moving in. In other words they're usually an expression of terror, though delight can also elicit them from some people with little self-control. Screams can serve to alert other's to one's plight and thus solicit help, but it's worth noting that in space, no one can hear them.
Screams were once used as the energy source of Piscis Magni, collected by specially trained Monsters known as "Scarers", until it was discovered that laughter is ten times more powerful. There's probably some deep significance in that.