What is offset in oscilloscope?
It's a feature on scopes. With offset it is possible to change the channel's reference level. You can do this by adjusting up or down relative to ground. You can use it for detecting small verying signals on a larger bias and it's also necessary when you want to subtract a certain DC voltage from the waveform.

You have also "position". Position adjusts the trace vertical position on the scopes display. Not to be confused with offset.

There is also a video which explain it here.