"The threshold is like how much sensory input is needed to excite a neuron. For example, like you said, if the neuron's threshold is 10, it will only get exited at ten. Not 7, not 9, not 9.5, ONLY 10. What actually happens is, if there is enough excitement to the neuron, the action potential (which is basically just a rush of ions) gets transmitted, and the neuron is 'excited'. But, if there isn't enough stimulation, the rush of ions doesn't happen, and so the neuron doesn't get excited."