The first would be announced when the ship was in the trough of a wave. When the waves rolled through, so would the ship from side to side, sometimes violently if the waves were big enough. We always tried to keep the ship's bow (nose) pointed toward the waves, or away from them, but we couldn't always do that for a variety of reasons. And when we couldn't, it could be a real problem.
They would announce: "Brace for impact," when we were about to hit something, like land or an iceberg or another ship. This never actually happened on any ship I was on but we ran drills for everything and that phrase came up in some of those drills. In which case you would have to grab something and hold on tight.
It's one thing to standby for heavy rolls and brace for impact externally, but it's quite another to do so internally. Another word for this phenomenon is "worry." Or "anxiety." And it can cause real health problems. What does such internal bracing accomplish? Does it help the situation in any way? Or does it merely cause physical discomfort?