2 Consider for a moment the case where there is negligable gravitational time dilation and length contraction. But this is just the coordinate frame energy density. Other elements have other interperetations. For instance T ii is a flow of momentum per area in the x i direction or the pressure on a plane whose normal is in the x i direction. T ij is the x i component of momentum per area in the x j direction or describes a shearing from stresses.
Any matter contained in the space provides contributions to the elements of the tensor.
In general relativity this must be expressed as a tensor equation.