Computers like Colossus, while electrical, and stored data electrically, were programmed mechanically using wires and switches and if you wanted to change the program being run on the computer, it would have to be re-wired.
This was a major problem, as everytime we needed the computer to do something different it required a rebuild.
Von Neumann, a Hungarian engineer and scientist, proposed storing the program (instructions – what tells the computer what to do) electrically along with data.
His implementation of a “Stored Program Computer” is called Von Neumann Architecture, and is the basis of the architectures computers use to this day.