What is it called when a row matrix is "flipped" into a column matrix?
Transposition.
Even if a row matrix and a column matrix are the same dimension and contain the same elements, they are considered different types. A "T" superscript is used when a column matrix is written as a row of numbers. The first element of a column matrix is the topmost (corresponding to the leftmost element when written as a row.)
The picture shows two different ways of writing out the same column matrix. To change an element, enter a new value into its box and hit enter. When the column matrix is written in a row the superscript "T" shows that it is really a column matrix. The elements of the column matrix have the same subscripts no matter how the column matrix is displayed.
A "T" superscript on column matrix with means to flip the column into a row, resulting in a row matrix.