In general, this means you've tried to assign something to a count, dimension or skip register that isn't (in TeX's view of things) a number. Usually the problem will become clear using the ordinary techniques of examining errors.
Two LaTeX-specific errors are commonly aired on the newsgroups.
The first arises from a misconfiguration in a system that
has been upgraded from LaTeX 2.09: the document uses
\usepackage{times}, and the error appears at
\begin{document}: the likely cause is lurking files that
remain from the LaTeX 2.09 version of the times package. The
times package in psnfss is a very simple beast,
but the LaTeX 2.09 version is quite complicated, and loads another
package: this is clear in the log if you have been "bitten" this
way. The resolution is to clear out all the old PostScript-related
packages, and then to install psnfss.
The second arises from attempting to use an example describe in The LaTeX Companion, and is exemplified by the following error text:
! Missing number, treated as zero.
<to be read again>
\relax
l.21 \begin{Ventry}{Return values}
The problem arises because the Companion's examples always
assume that the calc package is loaded: this fact is
mentioned in the book, but often not noticed. The remedy is to load
the calc package in any document using such examples from
the Companion.