The T command is used to calculate the transpose of a matrix: it flips a matrix along its main diagonal. This means that the (i,j)th element becomes the (j,i)th element, and vice versa. As a result, the transpose of an M by N matrix is an N by M matrix.
In addition to its many uses in linear algebra, the T operation is useful to programmers: with operations such as Matr►list( and augment(, which normally deal with columns, T allows you to use rows instead. See the "Related Commands" section for the commands that this is useful for.