Definition
Input validation is the practice of checking that data entering a system conforms to expected format, type, and range before you act on it. Validation happens at trust boundaries: when user input arrives at your server, when an external API response comes back, when a file upload is parsed. The alternative - trusting raw input - is the root cause of SQL injection, XSS, path traversal, and most other OWASP Top Ten vulnerabilities.
Validation is not sanitization. Validation rejects invalid input outright (a field that should be an integer and is not is an error). Sanitization transforms input to make it safe (stripping HTML tags from a message body). Good systems do both: reject data that cannot be corrected, sanitize data where the shape is right but some content needs escaping.
How SheetLinkWP relates to Input Validation
SheetLink Forms validates every form submission at several layers. The form plugin you use (Gravity Forms, WPForms, etc.) runs its own validation first. SheetLink then enforces size caps (message fields cap at 10,000 characters), strips binary payloads, validates email and phone formats where marked, and sanitizes all text through WordPress's sanitize_text_field before serialization. The backend /fanout and /two-way endpoints apply the same validation a second time so no malformed data ever reaches your sheet.