Hereto means “to this” — almost always “to this document, this contract, this letter.” It is lawyer-and-paper English, not spoken English. You meet it in set phrases: the parties hereto (the people this contract is between), attached hereto (fixed to this paper), agreed hereto (agreed to what this paper says). It points at the page in your hand, not at a place in the room. Nobody says it in a café. If you want the same idea in ordinary talk, you say “to this,” “to this letter,” or “in this contract.” It feels very formal, a little old, and completely at home in legal writing.

於此（法律／契約用語，指「對本文件」；日常不說）