In information theory, redundancy means that a message is encoded and tramsmitted using more bits that are necessary to emcode the message. If a piece of information is redundant, it can be left out, without loss of information. Redudant information can be used to reconstruct the original message, in the case errors occur in transmission, or storage. Operations like data compression reduce redundancy. This can be positive, as the data takes less space, but it can also be negative, because an error can no longer be corrected automatically.