Everyone has hang-ups - inhibitions, insecurities or situations that prevent one from having sex.
You have a partner, each is in the mood, you move closer in the direction of getting it on, and then everything stops. Someone's hang-up shows up.
Some hang-ups are mental – needing to have things arranged in a particular way - lights off, music on, lingerie or other “costuming,” grooming (brushing teeth, shaving, showered), focusing on having an orgasm in order to have satisfying sex, being over worried about STDs – the list goes on.
Some hang-ups are societal or based on religious beliefs – needing to be married, with a partner of a certain faith, or waiting until the third date. Perhaps you feel like you've had too many partners, and want to keep it below a certain number.