A cricket is like a tiny gable roof section which is installed behind a chimney (on the “roof side”) to keep water running down the roof from puddling up behind the chimney and potentially causing a leak. The presence of the cricket diverts water out from behind the chimney and off the roof.
Here’s how Barnhill Chimney installed a cricket on the chimney of a house with a metal roof in spring 2011.
This happens to be a metal chimney cricket, made to match the house’s metal roof. Crickets on shingled roofs are installed in a similar way, but of course they are topped off with shingles to match the rest of the roof.