"T" is a matrix that stores the nodal temperatures.
Here in the "T" matrix each row represents each time step. i.e. First row represents the intial condition of the rod, second row represents the nodal temperatures after the first time step... and the last row represents the nodal temperatures at the end time.
Each column represents each node. i.e. first column represents the first node, second column represents the nodal temperature of the second node and the last colun represents the temperature of the last node.
Ex: T
23
represents the nodal temperature of the third node after one time step.