Within math mode text would be not write as normal, because blanks were ignored and chars were printed curisve. A soultion would be this:
...
\begin{document}
some text $a^{2} + b^{2} \textrm{ text and chars within math } = c^{2} $
\end{document}
If the additional usepackage amsmath is used:
...
\usepackage{amsmath}
\begin{document}
some text $a^{2} + b^{2} \text{ text and chars within math } = c^{2} $
\end{document}
Hint: Just set a blank after "{" and one before "}".