It is always important, when answering questions such as these, to focus on what we already know from the play and what we don't know. The Prologue has effectively introduced us to a Verona that is plagued by the feud of two households, "both alike in dignity." We are told that two lovers from these households will get together but meet a tragic end. Then, as if to start off his play with an ultimate attention grabber, Shakespeare tranports us into the thick of the conflict as we see two servants from the house of Capulet joking about beating Montague men in a fight and dominating Montague women sexually:
Therefore I will push Montgue's men from the wall and thrust his maids to the wall.
As two servants from the Montague household enter, Sampson tries to insult their honour by biting his thumb at them, a highly insulting gesture. The conflict escalates from this point on as Benvolio tries to stop the fighting but his gesture is misinterpreted by Tybalt, who escalates it. Note the way that the brawl is linked explicitly to masculine honour, as it is shown to be full of physical and sexual bravado. The theme of masculine honour is clearly incredibly important, as in Verona a man is honour-bound to defend his honour whenever it is attacked. Of course, this becomes tragically significant when Mercutio and Tybalt fight.
The initial brawl scene thus introduces us to the feuding households, some of the key players, the concept of honour, and also shows us how tired of the violence the Prince is. All of the ingredients are introduced that will mark the rest of the play.