Thereto means “to that” or “to that place or thing.” It's an old-fashioned word you'll mostly find in formal writing, legal documents, or classic literature.

When a legal contract says “the parties agreed thereto,” it means the parties agreed to that (referring to whatever was just mentioned). If a document mentions “the rules and all amendments thereto,” it means the rules and all amendments to those rules.

You might encounter thereto in older books. In The Secret Garden, if Frances Hodgson Burnett had written “Mary walked to the garden door and the path leading thereto,” she would have meant the path leading to that door.

Today, we usually just say “to that” or “to it” instead. Nobody says “I'm going to the store and the parking lot thereto.” We'd simply say “the parking lot next to the store” or “the store's parking lot.” But understanding thereto helps you read historical documents, classic literature, and legal papers where this formal language still appears.