The purpose of this 110mm soil access pipe is to allow the soil pipe system to be inspected and cleared in the event of any blockage. It is usually situated at the foot of the soil pipe at the point where it enters the drainage system.
To use the access point, simply remove the access cap by unscrewing it from the threaded projection on the fitting. This could potentially release any blockage above the cap so it is always advisable to stand aside at this point.