DC offset refers to when a signal is offset away from the center zero point. The term comes from the field of electronics, where it applies to a direct current voltage. However, the concept has broadened to include any representation of a waveform. DC offset is defined as the mean amplitude of the waveform. There is no DC offset if the mean amplitude is zero.
DC offset is usually not a good thing. The offset takes up headroom, so any sound that has a DC offset will not be able to reach a peak volume when normalized or amplified. This issue has the potential to affect the entire mix. If a sound with DC offset is combined with a sound without it, the mix will have DC offset. It can also cause a clicking sound at points where audio sections are cut and pasted together as well as low-level distortion.
DC offset can be minimized in real-time through the use of a one-pole one-zero high-pass filter. If you already have the entire waveform, offset can also be removed by subtracting the mean amplitude from each sample. Very low frequencies are often referred to as “slowly changing DC.” While this is not technically accurate, it is easier for a high-pass filter to remove such a “changing offset” because its cutoff doesn’t reach as low a bandwidth as the above method.