Java's "cannot find symbol" error occurs when the compiler is unable to locate a symbol associated with an identifier due to issues like undeclared variables, out-of-scope references, misspelled identifiers, missing import statements, or incorrect object creation. This error arises because Java's compiler relies on symbol tables, which act as an address book, to track every identifier in a program. When identifiers are not found in these tables, the compiler halts, highlighting the symbol and its location in the code. The error message provides diagnostic information, helping to identify and resolve the issue by verifying declarations, checking scope and spelling, and ensuring necessary imports are present. Common cases include uninitialized variables, scope violations, typos, and missing imports, but less frequent errors can result from semicolon misplacement or missing the "new" keyword in object creation. Understanding these patterns and carefully reading error messages can simplify troubleshooting, while tools like Rollbar can automate error tracking and resolution.