Planer/Thicknesser Combination Machine
A planer thicknesser is a combination machine combining the functionalities of a surface planer (known as a jointer-planer in the US) and a thicknesser (known as a planer in the US).
A planner is a machine designed to produce a flat surface along a board's length. It can also be used to operate on the narrow edge of boards, preparing them for glueing into wider panels.
A thicknesser is a woodworking machine to trim boards to a consistent thickness throughout their length. This machine transcribes the desired thickness using the downside as a reference/index. So, to produce a completely straight planed board requires that the down surface is straight before planing.
Therefore by obtaining the first 2 flat sides that are perfectly perpendicular to each other using the planer and then thickness them to a consistent thickness throughout using the thicknesser. This results in a timber piece that is square to each other on all 4 sides and is often the first step performed when working with solid timber.