Bribery is unethical because it results in distortion of the marketplace. As a result, people who deserve to gain something actually gain very little or nothing.
Bribery means that someone gives another person something that they want in order to change the recipient's behavior. For example, if someone is running for office and they give people money in exchange for votes, this is a form of bribery. It gives the briber an unfair advantage, and the person not doing any bribing does not gain what he or she may rightfully deserve. This can also go the other way, with someone trying to bribe a person in power to do something for them. Appointed and elected officials cannot receive gifts because there is no way to definitively know the giver's intentions.