In carbon arrows, the term describes gluing a short piece, say an inch, of aluminum arrow shaft over the forward end of your arrow. It makes it stronger for impact,& helps keep the insert from crushing the carbon shaft when you hit something hard. Some put another smaller piece back by the nock of the arrow also. Makes for a very durable arrow, especially useful for surviving shooting stumps, rocks, etc.
If you do a search on this here, you will get a lot of info on how to do it and what you need for your arrows. There is a chart for what aluminum shaft fits over what carbon arrow.
In wood arrows it describes using a different heavier wood for the front part of the shaft, spliced to the lighter back part of the shaft.