A Close Couple Kit provides a seal between your toilet pan (the part you sit on ) and the cistern which provides the water for the flushing mechanism. Over time these can corrode and are a common point for leaks.
The cistern needs to be fitted at exactly 90 degrees to the toilet pan. This allows the large rubber ring washer, or “doughnut” as it is called, to be squashed flat between the two parts of the toilet. If the cistern is leaning over, even by a little, one side of the doughnut lifts and, with a lot of water pressure the seal can fail.