To be on tenterhooks (not tenderhooks) is to be nervously waiting to find out what is going to happen in a tense or perilous situation.
Literally, a tenter is a wooden frame used to hang newly woven woolen cloth in order to prevent it from shrinking as it dries. The tenterhooks, obviously, are the hooks on the tenter used to hold the cloth in place. The figurative sense, which developed in the late 18th or early 19th century, comes from the fact that cloth hung on tenterhooks is tense and stretched. Those adjectives might also describe the mental state of someone in anxious suspense over something.