Explain why p-nitrophenol is more acidic than phenol?

Nitro group of phenol produces - I and - R effect. Because of these two effects - $N{O}_2$ group is electron withdrawing in nature. So, the electron density in the O - H bond of p - nitrophenol decreases relative to the O - H bond of phenol.
The decrease in electron density of the O - H bond of p-nitrophenol, the polarity of O - H bond decreases. The electron withdrawing group (-NO ), withdraws electrons and disperses the negative charge. Therefore, -NO group stabilizes the phenoxide ion. Hence p-nitrophenol is more acidic than phenol.