Since mathematics (in general) should form part of the textual flow, one can follow an equation with symbol explanations. This would even include proper punctuation in the equation itself:
\documentclass{article}
\begin{document}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus ac velit dolor.
Nulla nunc dolor, commodo sed consectetur ac, dapibus malesuada odio.
Mauris porta libero eget nisi placerat quis bibendum tellus fermentum. It is therefore
possible to define the relationship
\[
x = R + E,
\]
where~$R$ is a racoon and~$E$ is an elephant. Sed suscipit tristique laoreet.
Nulla mi orci, rutrum sed dapibus sed, elementum nec lacus.
Phasellus id tellus mi, at rutrum justo. Nullam eget turpis justo, ullamcorper
pretium mauris.
\end{document}
This, in my opinion, is the best way to declare variables or symbols used in an equation. However, if need be, it is possible "to include a line under the equation saying what the variables mean." I'm not sure whether this is the "best way" though:
\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus ac velit dolor.
Nulla nunc dolor, commodo sed consectetur ac, dapibus malesuada odio.
Mauris porta libero eget nisi placerat quis bibendum tellus fermentum. It is therefore
possible to define the relationship
\begin{gather*}
x = R + E, \\
\text{where~$R$ is Racoon,~$E$ is Elephant}
\end{gather*}
Sed suscipit tristique laoreet.
Nulla mi orci, rutrum sed dapibus sed, elementum nec lacus.
Phasellus id tellus mi, at rutrum justo. Nullam eget turpis justo, ullamcorper
pretium mauris. It is therefore
possible to define the relationship
\begin{align*}
x &= R + E, \\
\text{where}~R &= \text{Racoon,} \\
E &= \text{Elephant}
\end{align*}
\end{document}
amsmath provides the mathematical alignment environments (align* and gather).
