Everything2 is composed almost entirely of nodes of one kind or another - this really just means that it is made of things that link to other things, so that mathematically speaking it is structured as a graph. Anyway, when people refer to 'nodes' here, what they often mean is the kind of node which holds writeups - in other words a heading, like 'Nodeshell', and everything that goes under it. Each of these nodes - technically known as e2nodes - can hold one or several writeups, or none at all. An e2node without any writeups is called a nodeshell.
Why do we have e2nodes without any writeups in them, if the main point of the things is to hold writeups? Well, the usual reason is because somebody thought the title would be good for putting things under. The main point of this is that e2nodes can hold soft links as well as writeups, which can be created easily by anyone with a user account. For instance, if someone notices that there is a glaring omission in Everything2's factual coverage of something, and they think they would like to write about it at some point (or see someone else write it up) they might create a nodeshell for it and link it to various relevant things which might help inspire the writer. Similarly, people sometimes create nodeshells because they think they have come up with a good title for a story or poem they would like somebody to write. Such nodeshells are prone to removal by administrators who disagree.
Another source of nodeshells is nodes in which all the writeups have been deleted. This is usually either because the editor or editors involved in the deletion hope that the node will one day be filled again, or else because they want to provide a firm link from an alternative title to the node in which relevant information is stored.
There is a list of recently-created nodeshells at Nodeshells, and a list of your own or somebody else's nodeshells can currently be found under 'Your Nodeshells'. Rude or very stupid nodeshells should be reported over at Nodeshells marked for destruction.