A chine log is what is used in chine log construction of a boat. It holds two pieces of plywood together by being the gluing surface between the pieces. This link shows what I mean, it also has a lot of other info about the building of a pd racer.

Here's a simple graphic to help. Imagine the yellow and green are pieces of plywood and you're looking at a slice of the joint from the end. The blue would be the chine log that provides more gluing surface.

The link mentioned above provides a much more comprehensive description and also some actual pictures of examples.