When the Emperor appears in a Tarot spread, he shows not only authority and law but also paternity and power. The Emperor is a war-maker, a leader, and represents the results of taking action. In some cases, he may represent a highly controlled sexual drive.
If the Emperor appears reversed in your reading, look out. He indicates emotional immaturity and an inability to "leave the nest" and separate from parents. This reversal is also connected to a loss of control, and symbolizes things that happen when one fails to take action at all but sits passively on the sidelines.