Leonato chats with his brother, Antonio, who is nearly bursting with gossip. Antonio reports that one of his men overheard Don Pedro and Claudio talking in the enclosed garden area.
Antonio reports that his man heard Don Pedro declare his love for Hero, and his intention to reveal his love for Hero that night. If Hero accepts Don Pedro’s love, then Don Pedro will tell Leonato right away, so they can be married, or something. (Of course, the man did hear Don Pedro say he'll woo Hero that night, but he missed the whole part where Don Pedro will be wooing on Claudio’s behalf.)
Leonato decides he’ll spring this news on Hero, so she can be prepared to deliver an answer. (Important motif introduced here: These guys are sticklers for action, not accuracy. Also, they’re sneaky.)