Chapters 61 and 62 are an extended flashback revealing how Rochefort (d'Artagnan's stranger from Meung) came to have a piece of paper with the town name "Armentières" written on it. The fact that the two chapters have been a flashback is not revealed until Chapter 62:
“Armentières,” [Milady] corrected [Rochefort].
“Write it down for me, in case I forget. There’s nothing compromising about the name of a town, is there?”
“We can’t be sure,” said Milady, “but I’ll take the risk anyway.”
She wrote the name on half a sheet of paper. Rochefort took it, folded it, and put it in the lining of his hat.
Rochefort and Milady are speaking at the convent where Madame Bonacieux has been hiding. The previous two chapters have detailed Milady's arrival there and Rochefort's visit, during which she makes a plan with him to meet up in Armentières in a few days' time. At the point when Rochefort puts the note in his hat, the reader will recall the events of Chapter 60. On the way to find Madame Bonacieux with his friends, d'Artagnan sees the man from Meung and points him out to Athos, Porthos, and Aramis. As the man rides away, someone comes out of the inn he just left with a slip of paper that has fallen out of his hat. D'Artagnan pays to see the slip of paper himself: it simply says, "Armentières." When Milady writes this town name on a slip of paper for Rochefort, it becomes clear that everything in Chapters 61 and 62 has filled in the events that led to that chance discovery of where the man from Meung (i.e. Rochefort) was going.
The two chapters at the convent have already been filled with intrigue because Madame Bonacieux reappears and Dumas gives the reader a glimpse into Rochefort and Milady's relationship. The revelation that these chapters have been a flashback is additionally exciting. Now, the reader realizes that Dumas left d'Artagnan and his friends two chapters ago at an urgent moment. It is unclear exactly how long after Rochefort left the convent he dropped the slip of paper, but the clock is certainly ticking on the musketeers' chance to reach Armentières in time to catch Milady, if everything goes according to her plan. By flashing back in time, Dumas creates the sense that the action of the story has paused at a critical moment. In this way, he keeps the reader wondering what will come next. This creates an immersive reading experience, which was useful when it came to retaining readers. The novel was first published in serial form in the newspaper, as many novels were in the 19th century, so Dumas needed to be mindful that readers didn't lose interest between installments.