Adding things up

A real neuron doesn't listen to just one thing. It receives signals from many neurons at once.

So it does something slightly less simple. It adds the incoming signals up, and fires only if the total is high enough.

"High enough" means it has a : a line the total has to cross. Below the line, the neuron stays quiet. Reach the line, and it fires.

Picture a neuron with three inputs and a threshold of two. One signal isn't enough. Two crosses the line, so it fires. That's the whole rule.