Null, in computing terms, refers to the absence of a value. It does not mean a value of 0, since 0 is itself a value, nor does it mean a blank space " ". It also does not refer to an undefined variable, which has not been created at all.
In the context of programming languages, a variable that has no value assigned is considered null. It may have been created without a value with the intention of adding one to it later, or it may have originally had a value that has been removed. A null variable can cause errors when the program tries to do something with that variable, expecting it to have a value.
In the context of a database, null is often used as a placeholder when a field was left blank, because that data was nonexistent or not available. For example, a database of customer information might include a column for a customer's phone number. If a customer doesn't share that information when the database entry is created, and nothing entered into that field, the phone number value will be null. That does not mean that the customer does not have a phone number, but that it is unknown whether they do and what it is. If the customer shares it in the future, it can be added to the field, replacing null.