Answer
$a_{ij}$ is the element of the matrix situated in row i, column j.
Work Step by Step
A matrix is a set of numbers, arranged into rows and columns, placed in brackets.
The numbers inside the brackets are called elements of the matrix.
$a_{ij}$ is the element of the matrix situated in row i, column j.
Example:
In $\left[\begin{array}{ll}
1 & 2\\
3 & 4\\
5 & 6
\end{array}\right]$,
$a_{21}=$... (row 2, column 1)..= $3, $
$a_{32}=$... (row 3, column 2)..= $6$
