Is your question refering to the "it" that is mentioned in the first stanza of this excellent poem? Let us remind ourselves of the context of this quote. J. Alfred Prufrock is walking through some rather shady streets of London thinking about what awaits him when he reaches his destination:
Streets that follow like a tedious argument
Of insidious intent
To lead you to an overwhelming question...
Oh, do not ask, "What is it?"
Let us go and make our visit.
The "it" thus relates to the "overwhelming question" that he feels driven to ask when he reaches his destination. However, the way that the poem instructs us not to ask what this question is and just focuses on the thoughts and feelings of J. Alfred Prufrock reflects his own fear or hesitation of asking such an "overwhelming question." Although the precise nature of this question is never exactly specified, we can infer that it is perhaps a marriage proposal to the woman that awaits him. Of course, his inability to even mention this question reflects his overall indecision and lack of sureness that this is the right thing for him to do.
Prufrock asks that the question that is at the center of what concerns him not be asked. He does not want to intellectualize or analyze his experience. He does not want to be objectified. He does not want to be psychoanalyzed and reduced to a number in the DMS 3. It is a mistake to read the poem as one would analyse a mental patient. Prufock fears intimacy because others have formulated him. Can we read the poem as refecting the modern world and not as psychological profile. To read it this way makes the reader one who puts Prufrock into a "formulated phrase". He does not want to discuss his feelings but to share his experience of the world.