when glaciers melt, they often have big chunks that break off or get isolated out in front of the glacier. The glacier is still melting, so there is still a lot of sediment being dumped by the glacier, so it keeps filling up the area in front of the glacier. However, there is a big chunk of ice sitting there, so the sediment cannot deposit where there is already ice.
Eventually that big chunk of loose ice does melt, but it leaves a hole in the ground where it was sitting. The hole is called a kettle (has a bit of a shape of a kettle, a rounded depression). If it gets filled with water, it is a kettle pond (or lake, if you choose to call it that)