Software via plug-boards

I don’t know if this is the right category, but can anyone please explain this line to me?

Software distribution has over the years gone through an enormous revolution. Originally, software was developed on things called ‘plug boards’.

Plain and simple, I don’t really understand it and would love to understand what they’re trying to say.

Software was developed on things called plug-boards? What does that mean, how did that work exactly? As opposed to now, you just download it, I can understand the concept but I don’t understand the sentence in whole.

Much appreciated!