Video editing (just one of the types of work we do) usually consists of copying the material you want from one place to another, with some resequencing, or simple shortening of the filmed material. That might be as computer files, or video signals on tapes. It's a time consuming process on a number of fronts:
This is an edit done as part of the decision making process. It's usually done on cheaper, lesser quality, formats than the final edit. It's typically done using copies of the material, so you can run it back and forth, repeatedly, without risking damaging the originals.
Offline editing is usually done for complex jobs where it'd be too costly to spends lots of in a high quality edit suit working out how the edit is going to go, and to avoid damaging your original material during the decision making, and the frequent alterations to those decisions.
Sometimes an offline edit won't be made. It might be adding too much time and running costs to the editing work to do an offline edit, then re-do it all again, for the online edit. The job might be straight-forward enough that you can go straight to the final edit.
This is a remake of the decisions made during the offline editing, but with the original material, at the best quality that's applicable to the job. This is usually quick to do, as the decisions have already been made. All that's generally necessary is to repeat the edits previously decided upon.