"The powers that be" refers to those people who are in charge, or in a position of authority. The authority figures might be part of a government, corporation, or in another position of leadership. The phrase often carries a somewhat negative connotation, implying that the people under the rule of the powers are resigned to accept their decisions, even arbitrary ones, whether they agree or not.
The phrase "the powers that be" is a plurale tantum, which is Latin for "in the plural only." This means that the phrase is only ever used in the plural form, and the singular phrase is never used. The powers that be can refer to just one person, however--it makes no difference in the phrasing whether the speaker is referring to one person or a group of people.
The phrase "the powers that be" was first used in the King James version of the Bible, in Romans 13:1. The direct quote is "Let every soul be subject unto the higher powers. For there is no power but of God: The powers that be are ordained of God." After that, the phrase began to be assimilated into daily language.
There are only a few ways to use the phrase in sentences. One example might be "The powers that be have decided we all need to work late today," or "The powers that be are considering my employment application." This phrase illustrates the power and control of the various authority figures, as well as the inability of the people being controlled to change the decision or debate it. It may also be used in a sarcastic or humorous context to suggest that an authority figure is incompetent.
The powers that be is a type of phrase known as an idiom, which is a frequently used, non-standard phrase, the meaning of which can only be gleaned from seeing the phrase as a whole, not the individual words. Because of the awkward phrasing of some idioms, they can be slightly difficult for non-native speakers to understand. In general, when speaking or writing, it is best to avoid the frequent use of idioms to prevent sounding cliched.