A sigmoid function is like a roller coaster. It starts out low, then gradually goes up and then down again. It looks like this in a graph:
![alt text](https://upload.wikimedia.org/wikipedia/commons/8/88/Logistic-curve.svg)
A sigmoid function is used a lot by computers. It helps them to figure out if something is true or false, or if something is "hot" or "cold". For example, a computer might use a sigmoid function to decide if an email is spam or not. It would use the information in the email, such as the number of words or the kind of words used, to decide if it is spam or not. It would then plot the information on the sigmoid function graph and decide if the email is spam or not, depending on whether its graph goes above or below the middle of the line.