The \dfrac macro renders fractions as those in displayed math, even if it is part of an inline equation, fraction, or other setting where LaTeX prefers to typeset an inline version.
Conversely, I frequently find myself using \tfrac, which renders fractions in an inline (or "text mode") fashion even if the setting is display mode, because I often find the large fractions distracting from what is important in my equations.
As with most useful but basic macros for mathematics, these are provided by the amsmath package.
Example code:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
Some fractions in inline math:
\( \dfrac{\sqrt 2}{2} \bigg/ \frac{\sqrt 2}{2} \bigg/ \tfrac{\sqrt 2}{2} \)
\vspace{1ex}
Some fractions in displayed math (in and outside of some environments):
\begin{gather*}
\begin{split}
\dfrac{\sqrt 2}{2} \bigg/ \frac{\sqrt 2}{2} \bigg/ \tfrac{\sqrt 2}{2} \ \\
\begin{bmatrix}
\dfrac{\sqrt 2}{2} & \frac{\sqrt 2}{2} & \tfrac{\sqrt 2}{2}
\end{bmatrix}
\end{split}
\qquad
\begin{cases}
\dfrac{\sqrt 2}{2} \\[2ex] \frac{\sqrt 2}{2} \\[2ex] \tfrac{\sqrt 2}{2}
\end{cases} \ \\
\end{gather*}
\end{document}
Result:
