Add notes in the equation under brace

Question

How to write the equation shown in the image

Answer 1

You could use \overbrace and \underbrace:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{equation}
    \overbrace{a}^{\text{output}} = \underbrace{b}_{\text{significant}} + \underbrace{c + d}_{\text{non significant}}
\end{equation}

\end{document}