A butt joint joins two pieces of wood flatly against each other using only glue. The end grain of one peice is set against the edge grain of the other.
Butt joints are rarely used beacuse it is weak and offers no support. Screws can offer some support, but they risk splintering the wood. If a butt joint is used, it’s ususally in a project that won’t be moved much or support much, if any, weight.