The threshold function can be seen in the following diagram:
Fig 2.13: The threshold function
On the x axis, we have the weighted sum of the input, and on the y axis, we have the threshold values from 0 to 1. The threshold function is very simple: if the value is less than 0, then the threshold will be 0 and if the value is more than 0, then the threshold will be 1. This works as a yes-or-no function.