A first-order RC low-pass filter.
Rendering
\usepackage{circuitikz}\begin{circuitikz}
\draw (0,0) node[ground]{}
to[V=$V_{in}$] (0,3)
to[R=$R$, -] (3,3)
to[C=$C$] (3,0)
node[ground]{};
\draw (3,3) to[short, -o] (4.5,3) node[right]{$V_{out}$};
\end{circuitikz}