Demyelinating lesions means areas of damage due to demyelination (the process by which the myelin coating around nerves is damaged and one of the key processes in MS).
Lesions of the optic nerve cause optic neuritis, so they are to be expected from your symptoms.
The tentorium cerebelli is a sort of membrane that separates the cerebellum from the cerebrum. The cerebrum is the biggest part of the brain, on the top. The cerebellum is the smaller, roundish part that lies underneath the back of the cerebrum. Supratentorial means above the tentorium cerebelli and basically means the cerebrum. Infratentorial means below the tentorium cerebelli and usually means the brain stem and the cerebellum. The brain stem is the part of the brain that lies at the top of the spinal cord, joining the spinal cord with the rest of the brain.
I’m not sure why the ophthalmologist has said infratentorial and then separated out the brain stem, but I would guess it means that you have lesions in both the cerebellum and the brain stem, as well as in the cerebrum.
The diagnostic criteria for MS include details on what needs to be shown on MRI. These are (for RRMS) having at least one lesion in at least two areas that are typical of MS, namely periventricular (next to the “lakes” of CSF in the middle of the cerebrum), juxtacortical (next to the outer layers of the cerebrum), infratentorial and spinal cord. The fact that the ophthalmologist has said that your MRI is consistent with MS makes me suspect that the lesions in your cerebrum are periventricular and/or juxtacortical. Together with the infratentorial lesions, this would satisfy the McDonald criteria.
It’s important to know that there are several different causes of demyelinating lesions, including some one off things, however the fact that you have developed symptoms since your MRI suggests that a one off thing is unlikely. MS is the obvious conclusion given the ON and the MRI results, however, an ophthalmologist is not qualified to diagnose MS with good reason: they are not neuros, so there may be another explanation and for now, if I were you, I would try and accept the likelihood of MS, but keep an open mind.
I hope the neuro appointment comes through soon so you can get some proper answers about what’s going on.