In literature, juxtaposition refers to placing passages of text close to one another in order to imply a connection or a contrast might be found between the two parts. I'm not sure what your teacher is after here, but there are several instances in the play when the juxtaposition of one scene with another leaves the reader with something to wonder about. For instance, Romeo's rant about how he'll never love another girl but Rosaline is juxtaposed with the scene where Paris is asking for Juliet's hand in marriage. Both scenes involve thwarted love: Romeo is stopped from getting near Rosaline because she will not be bought or sweet-talked; Paris is stopped from getting to marry Juliet because her father insists that she is too young and inexperienced. Viewers might wonder how the two scenes relate because it is not obvious the first time through, but the placement of scene 1 beside scene 2 suggests that there is some connection to be made.
In a different vein, Shakespeare also juxtaposes the marriage scene with the murders of Mercutio and Tybalt. The juxtaposition of these two scenes heightens the emotional intensity of the fight and viewers feel the conflicted emotions much more acutely than we would if the marriage and fight scenes were farther apart. Similarily, within the final scene, juxtaposing Juliet's awakening with Romeo's suicide not only increases the emotional intensity of the scene but also heightens the tragedy.
Juxtaposed means comparing two things side-by-side.
In Romeo and Juliet, one way to think of things being "juxtaposed" is Juliet's love for Romeo as compared, side-by-side, to her father and mother's desire for her to marry Paris. For Juliet, looking at the two choices closely, there simply is no comparison.