What happens
Paris, a young nobleman and kinsman of the Prince, asks Capulet again for Juliet's hand. Capulet says she's too young — not yet fourteen — and asks Paris to wait two summers. He invites Paris to a party at the Capulet house that night. Capulet's servant is sent out with a guest list and can't read it; he asks Romeo for help. Romeo reads the list, sees Rosaline's name, and decides to crash the party. Benvolio promises to introduce him to other beauties so he can compare.
Why it matters
The scene answers a question Act 1 Scene 1 didn't: how does Romeo, a Montague, end up at a Capulet party? The illiterate servant — a small comic accident — is the play's first glimpse of how easily one tiny piece of bad timing or bad luck lands two strangers in the same room. Shakespeare keeps using accidents like this to drive the plot. By Act 5 it'll be a quarantined letter doing the same work.
Capulet's dialogue with Paris is a small, careful piece of characterisation that comes back hard later. Here, in Act 1, Capulet is a thoughtful father: he wants Paris to wait, says she's too young, says "my will to her consent is but a part." This is the man who, by Act 3 Scene 4, will have moved her wedding up by a day and threatened to throw her in the street if she refuses. Shakespeare wants you to remember the gentler version, so the harsher one lands.
Thematically, the scene also names the play's question about how love works: Benvolio bets Romeo that if he sees other women he'll forget Rosaline. Romeo bets the opposite. Both turn out to be wrong, in the right way. He doesn't "compare" Rosaline to other women. He sees Juliet, and the comparison stops mattering.