Audio signals are conveyed electrically as an 'alternating current' or 'AC' signal. This might typically result in a signal voltage that varies, for example, between +3 Volts and -3 Volts, with a waveform that is more or less symmetrical about the 0V line. However, there are circumstances — often, but not always, as the result of a fault condition — where instead of alternating equally above and below zero volts, the signal voltage is offset, so it might now vary between, say, +4V and -2V. In this example it would have a DC-Offset of +1 Volt.
DC-Offsets are generally considered a bad thing, as they can result in audible clicks when editing the audio material, they can result in premature asymmetrical clipping (eg, postive peaks clipping before negative peaks), they can confuse level metering systems, and they can even cause damage to loudspeakers and headphones in some specific circumstances.
As the DC-Offset 'signal' effectively has a frequency of 0Hz, a DC-Offset can be removed with a high-pass filter set to a few Hertz (typically 3 or 5Hz, but anything with a turnover below 20Hz would get the job done).