Someone who's faint-hearted is fearful or frightened. If a haunted house advertises itself as "not for the faint-hearted," it's probably really scary.
If you're generally faint-hearted, you tend to be a little timid or anxious, easily scared or overwhelmed. A faint-hearted person might choose not to go on the steepest roller coaster or watch the scariest horror films. Faint-hearted was first used in the fifteenth century, and it combines the original meaning of faint, "lacking in courage," with hearted, or "at heart."