The error
comes in reaction to you giving LaTeX a! LaTeX Error: There's no line here to end. See the LaTeX manual or LaTeX Companion for explanation.
\\ command at a time
when it's not expecting it. The commonest case is where you've
decided you want the label of a list item to be on a line of its own,
so you've written (for example):
\begin{description}
\item[Very long label] \\
Text...
\end{description}
\\ is actually a rather bad command to use in this case (even if
it worked), since it would force the 'paragraph' that's made up of the
text of the item to terminate a line which has nothing on it but the
label. This would lead to an "Underfull \hbox" warning message
(usually with 'infinite' badness of 10000); while this message doesn't
do any actual harm other than slowing down your LaTeX run, any
message that doesn't convey any information distracts for no useful
purpose.
The proper solution to the problem is to write a new sort of
description environment, that does just what you're after. (The
LaTeX Companion offers a
rather wide selection of variants of these things.)
The quick-and-easy solution, which avoids the warning, is to write:
\begin{description}
\item[Very long label] \hspace*{\fill} \\
Text...
\end{description}
which fills out the under-full line before forcing its closure. The
expdlist package provides the same functionality with its
\breaklabel command, and mdwlist provides it via its
\desclabelstyle command.
The other common occasion for the message is when you're using the
center (or flushleft or flushright)
environment, and have decided you need extra separation between lines
in the environment:
\begin{center}
First (heading) line\\
\\
body of the centred text...
\end{center}
The solution here is plain: use the \\ command in the way it's
supposed to be used, to provide more than just a single line break
space. \\ takes an optional argument, which specifies how much
extra space to add; the required effect in the text above can be had
by saying:
\begin{center}
First (heading) line\\[\baselineskip]
body of the centred text...
\end{center}
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