Zero is the number separating the negative and positive numbers. Zero can mean many things. For example we can say there are zero things or we can say the net force on an object is 0. Both of these mean different things. When we say there are zero things we me that numerically there are none but when we say the net force on a object is 0 we mean that the sum is zero. This means that forces could still be pushing or pulling on an object.