How can i have text between brackets in equation with respect to some criterion?

(edited answer after noticing that I had misunderstood the OP's overall objective)

Something like this?

\documentclass{article}
\usepackage[margin=1in]{geometry}
\usepackage{mathtools,ragged2e,array}
\newcolumntype{C}{>{{}}c<{{}}} % column type for binary and relational operator symbols
\newcommand\mymat[2][0.12]{%
  \begin{Bmatrix}
  \footnotesize
  \begin{tabular}{@{}p{#1\textwidth}@{}} % default width is 0.12\textwidth
  \itshape\Centering #2
  \end{tabular}
  \end{Bmatrix}
}
\begin{document}
\begin{equation}
\setlength\arraycolsep{0pt}
\begin{array}{cCcCcCc}
\partial_t C 
& + & \partial_x u C + \partial_x v C 
& = & D_x \partial_{xx} C + D_y \partial_{yy} C 
& + & R(C) \\[2ex]
\mymat{Time rate of change of concentration of chemical component}
& + &
\mymat[0.1]{Term of advection of chemical component}
& = &
\mymat[0.1]{Change in component due to diffusion}
& + &
\mymat[0.14]{Rate of formation of component~$-$ Rate of consumption of component}
\end{array}
\end{equation}
\end{document} 

I guessing that you like text in square brackets related to above equation. For this people usual use something like this:

\documentclass{article}
\usepackage{geometry}
\usepackage{mathtools}
\usepackage{newtxtext,newtxmath} % optional
\usepackage{ragged2e}
\usepackage{array}
\usepackage{stackengine}
\stackMath
\newcommand{\underlabel}[3][9em]{\stackunder{#2}
                                            {\overbrace{\scriptsize
  \begin{tabular}{@{}>{\Centering}p{#1}@{}}#3\end{tabular}}_{}}%
                                }
\newcommand{\mystrut}{\vphantom{\partial_{yy}}}

\begin{document}
    \begin{equation}
\underlabel{\partial_t C\mystrut}{Time rate of change of concentration of chemical component}
    \ + \ \underlabel{\partial_x u C + \partial_x v C\mystrut}{Term of advection of chemical component}
    \ = \ \underlabel[11em]{D_x \partial_{xx} C + D_y \partial_{yy} C}{Change in component due to diffusion}
    \ + \ \underlabel[11em]{R(C)\mystrut}{\tabcolsep=3pt
        \begin{tabular}{@{}m{5em} c m{5em} @{}}
                        Rate of formation of component
                        &$-$& Rate of consumption of component
        \end{tabular}}
\end{equation}
\end{document}