In this reaction, aniline (C$_6$H$_5$NH$_2$) is treated with sodium nitrite (NaNO$_2$) and HCl at low temperatures. This leads to the formation of a diazonium salt intermediate, which then reacts with phenol (C$_6$H$_5$OH) to form a nitro derivative (NO$_2$). The product Y is therefore the nitro derivative of the benzene ring.