A diode matrix is a two-dimensional grid of wires, where each "intersection" where one row crosses over another either has a diode connecting them, or the wires are isolated from each other. It is one of the most popular techniques for implementing a read-only memory.
A diode matrix is used as the control store or microprogram in many early computers. A logically equivalent transistor matrix is still used as the control store or microprogram or decode ROM in many modern microprocessors.
At any one instant, a single row of the diode matrix (or transistor matrix) is activated. Charge flows through each diode connected to that row. That activates the column corresponding to each row. The only activated control signals during that instant were those whose corresponding column wire was connected with a diode to that row.