Servants of the Capulet and Montague houses start a fight. Benvolio sees them and rises his sword to stop the fight. On the other hand, when Tybalt sees Benvolio he immediately prepares to attack. This shows how opposites Benvolio and Tybalt are: Benvolio is a more rational, calmed man whereas Tybalt is impulsive and prone to fight.
An example of how they are foil characters can be found in their first interaction. Benvolio says to the servants,
Part fools! Put up your swords; you know not what you do
attempting to separate them and asks Tybalt to"keep the peace" to which Tybalt replies
What, drawn, and talk of peace! I hate the word, As I hate hell, all Montagues, and thee