Mirrors are powerful symbols in all cultures. They let us see ourselves for who we truly are and help promote contemplation of what we have become. It's no surprise that the mirror - and the ba gua mirror in particular - is important in feng shui.
The term ba gua refers to the standard eight trigrams, or three-line combinations, that are found in the I Ching and feng shui. So a ba gua mirror is an octagonal mirror (8 sided) that has those main eight trigrams circling around it.
Depending on where the mirror is placed, its purpose can be to allow further examination of the space in question, or to reflect away a negative energy.
For example, many people have a mirror somewhere near their main entrance, so they can ensure they are neat and orderly before they head out to face the world. The mirror would not want to face the door, but a position on a side wall can make it useful and helpful.
For use in warding off negative external energy, you would want to get a sturdy, weather resistant ba gua mirror and put it on the OUTSIDE of your home facing that negative force. So, for example, if you had a cemetery next door, you would put the mirror facing that cemetery. This helps to keep that negative energy at bay.
I'll note that some people feel that a mirror should never, ever be used inside the house, and only placed outside the house. I resist saying "never" in any aspect of life. There are far too many variables in life to say that "never" could be appropriate. We are all different people. We all react differently to symbols and to objects. Yes, some people might not react well to a mirror in their home - but others might find it is absolutely perfect and brings them great joy and serenity. So in the end it's important to listen to your own heart.