What does "too many unprocessed floats" mean?

There are about 2 pages in the LaTeX book about what the "too many unprocessed floats" error means. The short answer is that you have put more figures in at a greater rate than LaTeX can put them down nicely on a page, and it has had to save more of them up than it can hold. There are two possible solutions:

1. spread your figures further apart in your document so LaTeX can fit one at the top of each page (or one at the top of each column) like it thinks is pretty.
2. put in a \clearpage or \cleardoublepage command to allow a page full of figures to be generated.