What is meant by "supplicating to the deceased" mentioned in the context of prohibiting the taking of prophets' graves as mosques and considering that among the causes of polytheism?

Visiting graves is of two types: permissible and innovative (bid'ah). The permissible visit is to visit the graves of Muslims with the intention of supplicating for them and remembering the Hereafter. It is also permissible to visit the graves of disbelievers solely to remember the Hereafter. As for the innovative visit, it includes seeking needs from the deceased, or seeking supplication and intercession from them, or supplicating at their grave believing that it is more likely to be answered. These actions are innovative and akin to polytheism (shirk). Similarly, taking graves as mosques is forbidden and a cause of Allah's wrath. Seeking intercession through the deceased or through their person is an innovation (bid'ah), but seeking intercession through faith in them and love for them is permissible.