JavaScript's "ReferenceError" occurs when a variable is used without being declared, often due to missing the 'var' keyword or attempting assignments outside the current scope, especially in strict mode. In non-strict mode, such errors are ignored, but they highlight potential coding issues that can disrupt function execution. The text illustrates this with a code example where omitting 'var' leads to an error, while including it resolves the issue. It also mentions various other ReferenceErrors, which can arise from undeclared variables, deprecated usages, and invalid assignments. Saif Sadiq, a Product Growth specialist at LambdaTest, provides insights into these common JavaScript errors.