The CWS (control wheel steering) button varies on the actual implementation but basically it enables the AP to be overrided and the plane to be controlled manually.
With some autopilots it has other functions too. For example with the KFC225 if you are flying in PIT mode then the CWS button can be used (together with a manual pitch change) to modify the pitch which the AP will be maintaining when CWS is released. Similarly with altitude hold - the altitude which was current at CWS release will be the new target altitude (IIRC).