Being on tenterhooks is a phrase that tends to mean you are anxiously or tensely waiting, or merely anxious. This usage showed up in the 1700s and was well understood at the time. Today, you might use the expression and be fully understood, but people may wonder what tenterhooks actually are, since they are no longer in common use.
The initial use of the modern idiomatic phrase was metaphoric. Being on tenterhooks referred specifically to the process of cloth making employed in Europe as early as the Middle Ages. After cloth was milled and washed, it had to be dried, but this posed a problem. Normally, drying meant the cloth would shrink significantly, which was undesirable. Cloth makers would make less profit if the cloth shrunk, since cloth was sold by lengths. Less cloth after the drying process translates to less money for the cloth maker. To avoid losing profits and valuable cloth, during the drying process cloth was tented to prevent it from shrinking.
Tenting meant the cloth was stretched out or suspended, like a tent, and usually attached to hooks, often simple nails, that were, as you might guess called tenterhooks. Thus cloth on hooks was stretched tight to dry. This sped up the drying process and prevented the cloth from shrinking. The actual tenting process was usually accomplished via small or large devices called tenters, usually wooden frames upon which the cloth would be attached to the hooks.
Tenters are no longer in common use, and as such, tenterhooks have largely disappeared from use. The closest modern comparison might be actually using a tent, which is normally canvas or nylon stretched over a frame and then hooked or attached to keep stable. Although the name tenterhooks or tenters are no longer used, the current expression survives the old process.
As such, a person on tenterhooks is stretched tight and waiting, translating to anxiousness. Someone waiting for a job offer, the answer to an important decision or a sequel to a book may be said to be waiting on tenterhooks.