What does Waste Ink nearly full mean? Canon MP370

If you peel enough apples, and throw the "waste" apple-peel into the basket, eventually the basket gets nearly full, and must be emptied.

Similarly, if you print enough pages, some of the ink becomes "waste",
and that "waste-ink" goes into a small sponge-and-bucket apparatus.

So, take the printer to a qualified technician, and get the printer serviced.