A leader/ender is something to start the chain piecing. Some keep small squares next to the sewing machine. When they are ready to start a chain of (whatever) for the current project, they piece a light and dark (same size) square together, and do the same for the end. That replaces the 'thread bunny' used at the beginning/end of a chain. When all the leader/enders are pairs, they piece two pairs together for leader/enders, etc. Eventually there are lots of 4 square groups that can be put together for a scrappy quilt. (This is just one example of the leader/ender idea.) On Quiltville.com there is more information.