Null is not a very common concept in common language because it means "nothing" in a way that is hard to fathom. Zero does mean nothing but it is still a definite number. Null is an absence of any value.
In English you might hear it used in the phrase "null and void" (which means something is invalid). The two words literally mean the same thing. Null is a void. A total absence of anything.
In my experience Null is most commonly used in programming. Consider a Boolean variable. Something is True or False. 1 or 0. But what a value isn't provided? Null serves as a "no comment". Something that is neither True or False. Being able to make this distinction is important because Zero and Null are interpreted in similar ways. More than a few computer bugs have occurred due to not handling null values correctly (incorrectly assuming they meant False).
In a similar vein but irrelevant here, C-derived programming languages also terminate basic strings/char-arrays with the ASCII "null character"