Recall that a matrix is a list of lists with same size.
The prefixed function at or the index notation [...] is used to access to an element or a row or a column of a matrix:
This may be used to extract a full column, by specifying all the rows
as index interval.
You may also assign an element of a matrix using the index notation, if you assign with := a new copy of the matrix is created and the element is modified, if you assign with =<, the matrix is modified in place.