A wood dowel, often referred to a dowel pin, is a cylindrical piece of wood used to join two pieces of wood together. A wood dowel acts just like a floating tenon except that dowels have less gluing area and use multiple holes (mortises) that must maintain alignmein mating pieces. Which can be tricky without a proper doweling jig (see: Note: What Is a Jessem Doweling Jig?).
You can purchase pre-made dowels designed specifically for wood joinery that can come in various diameters, lengths, and special grooves (fluted and spiral groove) to help with dispersing glue and increase glue area. The most common dowel diameters you can purchase are: 1/4″, 3/8″, and 1/2″. These are the most common sizes that will work for most wood working projects.
You can also make your own dowels with similar species of wood you are using or you can use a hobbyist dowel which tends to be smaller and not be even in diameter.