In SQL, a NULL value signifies that a particular data field doesn't hold a valid or meaningful value. It's not the same as an empty string or zero. Think of it as a placeholder indicating the absence of data. This is different from a zero, which represents a specific value. NULLs are often used to represent missing information, such as a customer's phone number that hasn't been provided yet. Understanding NULLs is critical for accurate data analysis and manipulation. For example, if you're calculating the average income of customers, a NULL income value will skew the result if not handled correctly. SQL provides specific functions and operators to handle NULL values effectively, preventing unexpected results.