Left temporal floor arachnoid cyst is not a serious condition. Arachnoid cyst is a relatively common benign intracranial lesion, which is cystic in nature and contains cerebrospinal fluid, with a clear border between the lesion and the surrounding brain tissue. The left temporal floor is a non-functional area of the brain, and there are no important functional brain tissues or vascular nerves. Therefore, left temporal floor arachnoid cysts are not serious and usually do not have any clinical symptoms and do not require any special treatment, and should be reviewed regularly.