What does "heart misses a beat, one's" mean?

Also, one's heart skips a beat or stands still. One is startled, frightened, or very excited. For example, Her heart missed a beat when she heard her name called out in the list of finalists, or When the bear appeared in front of us, my heart skipped a beat, or My heart stands still at the very thought of flying through a thunderstorm. All these hyperbolic expressions can also be used with make, meaning “to cause one to be startled” as in That blast from the ship's whistle made my heart skip a beat.