To cool one's heels is to be kept waiting. If a person has to cool their heels for too long before an important job interview, they're bound to be extra nervous by the time they're summoned.
There's an implied discourtesy (and impatience) in cooling one's heels — if you're on time for a meeting and the other person makes you cool your heels, they're being a bit rude or dismissive. This phrase dates from the 17th century, defined as "wait in attendance at a great man's door." Etymologists think the cool part comes from the sense of resting one's feet after a long walk.