As you probably remember, we have seen before that Wemmick has something of a castle where he lives. It is not that big of a castle but it is made to look like one, has a moat, and a cannon and a drawbridge and things like that. He lives there with his father (or Aged Parent or even Aged P).
What we find out in this chapter has to do with the castle. We find out that he has rigged up a lever that he can pull outside the castle. It will open a little wooden flap near where his father hangs out and it will say "John" on it. When Aged P sees this, he knows that he has to let the drawbridge down to let Wemmick in. There is one like it for Wemmick's lady friend, Miss Skiffins.