What is the meaning of Imam Ibn al-Jawzi's saying: "Do not antagonize the awliya' (friends) of Allah, for you sleep while they are awake, and they may supplicate against you, and it may be answered concerning you, while you are unaware"? And can a deceased person supplicate against a living person?

Every pious believer is a friend (wali) of Allah, whether alive or dead, in accordance with His (the Almighty's) saying: "Unquestionably, [for] the friends of Allah there will be no fear concerning them, nor will they grieve. Those who believed and were fearing of Allah." The intent of Ibn al-Jawzi's words is that whoever shows enmity towards the living friends of Allah, they may supplicate against him, and Allah will answer their supplication. So, Ibn al-Jawzi warns against showing enmity towards the friends of Allah Almighty, because its consequences can be dire.