To understand what it truly means to be a brain in a vat one must understand the meaning of these words. The proper definition of "Vat" is a large vessel, tube, cistern, or barrel used to hold or store liquids. So essentially what it means to be a brain in a vat is to be a brain in a vessel with sustaining liquid and neural connections. Neural connections mean electronic or organic connections from your brain to a main frame. The main frame would most likely be a computer, but the connections would input and output signals from your brain to the main frame. Since the connections would be flawless and the brain would continue to operate one would not know the difference whether or not one is a brain in a vat.
The setup would be so perfect that we would have neural connections making our brain think or believe that we have organs, arms, legs, feet, and hands. It would basically create a feel and image in our brain for these body parts and allow one to feel and use them. The input would be connections to sensory organs making one believe and feel as though one has organs. The output would be connections to our "imaginary" muscles. That way one would feel as though one had a body with organs and muscles. The setup would be so perfect that if one were to be put in a vat at this current moment that the brain will not be able to tell a difference.
Every connection would be connected to a computer creating the feeling that one would have every organ and muscle that a natural human being would have. To be a brain in a vat would essentially to say that one is in an eternal dream. It would mean that your body or what you believe to call a body is unreal. All it is is an image created by your brain through electro magnetic impulses to give you the feel of arms, feet, hands, and legs. The possibility of being a brain in a vat gives one reason to lack sure knowledge about the existence of ones body parts. If one is a brain in a vat in an "imaginary world", take The Matrix for example, then the brain could technically break rules created by the system and / or the brain could get strong enough to begin to realize its true existence.
Therefore one may believe that one may have ideas as to how he / she may be a brain in a vat. Being a brain in a vat would mean to live an imaginary life in the dream state the brain is in or just being a brain in a vat having its ability to produce so much energy to render it to other such machinery. This is a possible reason to believe why one is a brain in a vat and what it exactly means to be a brain in a vat if one is in fact a brain in a vat.