#T_{mu nu}# is the energy-momentum tensor.
It is a 4x4 tensor where each element represents a flux in a direction of momentum in a direction.
To explicitly state things, we have the normal 4 coordinates:
#x_0 = t, x_1 = 'x', x_2 = 'y', x_3 = 'z'#. Here, momentum in time is just energy (you can use units to justify this if you wish).
The #mu# coordinate represents the direction of change. For example, #mu = 0# is all of the changes of values over time. #mu = 1# is the changes across #x# values.
The #nu# coordinate represents the direction of the momentum. #mu = 0# is the energy, #mu = 1# is the momentum in the #x# direction, etc.
From this, we construct a whole grid with the following significances:
#T_00# is the energy change over time
#T_01# is the energy change in the first spatial direction ("x")
#T_02# is the energy change in the second spatial direction ("y")
#T_03# is the energy change in the third spatial direction ("z")
#T_10# is the momentum in x change over time
#T_11# is the momentum in x change in the first spatial direction
#T_12# is the momentum in x change in the second spatial direction
#T_13# is the momentum in x change in the third spatial direction
and so on.
