When you are asked to "trace and comment" on something, you are being asked to find all, or least most, occurrences of the element and then to comment on it. When commenting, you will want to ask yourself and address questions such as: How does this line/passage refer to effeminacy? What does it show about the characters that the line/passage is referring to? What does the reference to effeminacy say about the play's themes?
In order for you to understand this prompt, it may also help you to understand what a "motif" is and what "effeminacy" is. A motif is any element in literature that is very obviously being repeated. It can be a literary device, a symbol, a reference, a sentence structure, a verbal construction, or just about anything. The word "effeminacy," or "effeminate," refers to a masculine person demonstrating character traits that are conventionally accepted as feminine. Such character traits can be tastes, such as clothing tastes, habits, or even emotions (Random House Dictionary).
All throughout the play Romeo is accused of acting effeminately, especially in several scenes in Act 3. We even see Romeo accuse himself of being effeminate in Act 3, Scene 1, after Mercutio is stabbed by Tybalt, when ironically, Mercutio earlier accused Romeo of being effeminate in Act 2, Scene 4, after Romeo returns from being out all night. We see Romeo reciprocate and accuse himself of being effeminate in the lines,
O sweet Juliet,
Thy beauty hath made me effeminate
And in my temper soften'd valour's steel! (113-115)
Romeo's argument is similar to Mercutio's argument: Romeo's passionate, uncontrolled feelings of love have made him mentally, physically, and emotionally weak. We see that while love may be a good thing, these uncontrolled feelings are not a good thing, as they lead to destruction, namely, at least in part, Mercutio's destruction, Romeo's own destruction, and Juliet's. Hence the recurring motif of effeminacy relates to Shakespeare's theme of uncontrolled emotion, and he is using both the recurring motif and the theme to make a philosophical point about the dangers of uncontrolled emotions, even love.