What does it mean to "complete" a well? For sure, if we're going to complete the well, that means we're not done with what we ought to do.
After going through the trouble to explore and drill a well, the next step is not to start producing. The well has to be completed first in order to optimise production and ensure surface control of what's going on in the well.
Well completions is to an oil/gas well what icing is to a cake. It may not look like much on the surface but it's really crucial.
For example, casing and cementing the well to make it easy to control production from the well, perforating the well to open up the reservoir to producing through the casing. All these fall under well completions.
There are other things to do but these are just a few. Well completions is a very important aspect of the production process.