A footprint is an indication of the popularity of a node. The more often a node is visited, the more visible the footprint becomes. This is most useful in multi-user systems, where it contributes to a sense of community. Just as with breadcrumbs, it is important that the footprint fades slowly with time. Otherwise, there is an inexorable tendency towards all nodes being marked at maximum intensity.
Note that this footprint tool differs from that discussed by Nielsen. However, he briefly mentions a "voting filter", which displays only those nodes which have been sufficiently popular with previous readers (Hypertext and Hypermedia, 141).