"Better the devil you know" is an English idiom. It is used to suggest that, when faced with two choices, it is best to stick with something familiar even if it is unpleasant. This is because the alternative, which is unknown, could be much worse than the current situation. When using this idiom, many speakers often complete the phrase by saying, "Better the devil you know than the devil you don't." The phrase gets its power from the negative connotations associated with the devil, a figure generally considered to be the personification of all things evil in cultures where Christianity is common.
If a person uses a phrase that has become popular because the use by many people over a long period of time, he or she is using an idiom. An idiom generally takes some situation that is a bit larger than life and uses it in a metaphorical way to describe some actual situation. Many of these idioms can be considered proverbs, since they aim to give advice to the listener.
When using the phrase "better the devil you know," people are referring to the choice between two alternatives. The first option is something that the person making the choice has already experienced; he or she likely knows it to be somewhat distasteful or difficult. By contrast, the other alternative is something that may or may not be superior to the familiar choice. Someone using the phrase is essentially saying that the unfamiliar choice is a greater risk than what the person is already experiencing.
For an example of a context which would suit the phrase, someone can imagine a man who has an opportunity to get a new job. His current job features a boss who annoys the man, but, by the same token, the man has learned to deal with him on a daily basis. By contrast, the new job may bring up any number of new issues that could be much more harmful to the man than the annoying boss. The boss is the devil he knows, while the unexpected situation surrounding the new job is the devil he doesn't know.
What makes the phrase "better the devil you know" such a powerful one is its inclusion of the devil to describe the two situations. This is an obvious exaggeration for effect, since the devil is associated with eternal torment and infinite misery. The situations that cause this phrase to be uttered are not nearly that bad, but the phrase hints at the possible unpleasantness involved.