The simplest possible activation function is the threshold function. The output of the threshold function is binary: 0 or 1. It will generate 1 as the output if any of the input is greater than 1. This can be explained in the following diagram:

Note that as soon as there are any signs of life detected in the weighted sums of inputs, the output (y) becomes 1. This makes the threshold activation function very sensitive. It is quite vulnerable to being wrongly triggered by the slightest signal in the input due to a glitch or some noise.