I received this answer by Dan Craciun on experts exchange which answers it for me:
In geddy, a partial (denoted by a _ in front of the name) is a reusable file/template, that is meant to be used in more than one template. It helps share code between different templates.
See here for a longer explanation (about half page, "An intro to partials"): http://code.tutsplus.com/tutorials/creating-a-todo-list-app-with-nodejs-and-geddy--net-24535
HTH, Dan