This is a theoretical model of a neuron propsed by neurobiologists Warren McCulloch and Walter Pitts in 1943. The McCulloch-Pitts model says that a neuron acts like a binary switch (like a transistor) -- either off or on depending upon the input from the neuron's dendrites.
The model proposes that a neuron performs threshold logic. For example, if a neuron's threshold is three, then three or more of its inputs must be active in order for the neuron to to be active.
For obvious reasons, the McCulloch-Pitts model is extremely attractive to computer scientists; but it been replaced in general neuroscience by models which look at the neuron as an integrator, rather than a simple switch.