A kettle is a type of geological formation left behind by glaciers. Glaciers tend to dump a lot of junk (called till) when they melt -- rocks, gravel, sand, and, sometimes, really gimongous chunks of ice. Sometimes these chunks of ice are buried in the till, and when they eventually melt they leave giant holes and craters in the ground. These holes often fill with water, leaving the landscape dotted with small lakes -- kettle lakes.
Kettles are also known as kettle holes or ice-block lakes. If you live in the UK, many lochan are actually kettle lakes; if you live in the American mid-west, you probably call kettles 'potholes'.