1

This question already has an answer here:

How to write the equation shown in the imageadding notes under the term in a mathematical equation

marked as duplicate by user13907, Przemysław Scherwentke, Jesse, Heiko Oberdiek, Claudio Fiandrino Aug 22 '15 at 10:30

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3

You could use \overbrace and \underbrace:

enter image description here

\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}

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