Sir Andrew Aguecheek is Toby Belch's super-wealthy drinking buddy. Like Toby, Aguecheek is always down for some fun and spends most of his time drinking, singing, and dancing, which helps create the play's festive atmosphere. Aguecheek is Toby's out of town guest and he's hoping to make Olivia "Mrs. Andrew Aguecheek." He doesn't have a chance, of course, but Toby convinces him to stick around and have another go at the Countess in between beer guzzling sessions.
Aguecheek is happy to stay and play, but when he complains that Olivia seems more interested in "Cesario," Toby convinces him to pick a sword fight to prove his "manhood" and his love to the Countess. Not being very bright, Sir Andrew falls for Toby's line and immediately begins to posture. Toby even gives Aguecheek some pointers about how to act like a "swearing," "swaggering," sword-drawing brawler. Sir Andrew gets into the role but immediately backs down when he thinks "Cesario" is ready to rumble.
The scene is hilarious, of course, and most of Aguecheek's job is to be a comedic figure. We can also see that he aborted duel scene between "Cesario" and Aguecheek is an important moment in for the way it comments on the ambiguity of masculinity. Like "Cesario's" costume, Sir Andrew's impersonation of "manhood" demonstrates that "manliness" can be performed and faked, just like any other social role.