I have a lesion in the right temporo-occipital that are likely covernoma vascular malformation. what does it mean and is it harmful?

No: Mostly these cavernous sinuses are benign and for cosmetic purpose you can get laser or surgery for them . Sometimes they regress by themselves.

Usually unharmful: Cavernous malformations or cavernomas, are a type of vascular abnormality of the central nervous system, consisting of a cluster of abnormal, dilated vessels. People are typically born with a tendency to develop them, but are usually benign and most everyone do not have any symptoms. They are usually not harmful and have a very small risk of bleeding usually on range of 1-2% per year.