You have an arachnoid cyst. That is a cyst in the lining of the brain. Often people are born with these.
The next part shows damage in a specific part, the sub cortical centrum semiovale bilateral. You neurologist will have to explain what impact that has, if any.
Polyp are often found in the sinus.
You will have to see a neurologist who will evaluate you and determine next steps. I cannot really tell you more based on the information you have given.
Taken from various free online dictionaries, medterms website, etc.:
infratentorial= beneath the tentorium of the cerebellum.
tentorium= a fold of dura mater which covers the cerebellum. It supports the occipital lobes of the cerebrum
cerebrum= The largest part of the brain, divided into two halves or hemispheres.
occipital lobe= The rearmost lobe of each cerebral hemisphere, containing the main visual centers of the brain.
Cisterna magna= a large subarachnoid space between the caudal (posterior) part of the cerebellum and the medulla oblongata
Medulla oblongata "the base of the brain, which is formed by the enlarged top of the spinal cord. This part of the brain directly controls breathing, blood flow, and other essential functions)."
From emedicine medscape:
"Arachnoid cysts are benign cysts that occur in the cerebrospinal axis in relation to the arachnoid membrane and that do not communicate with the ventricular system. They usually contain clear, colorless fluid that is most likely normal cerebrospinal fluid; rarely, they contain xanthochromic fluid. Most are developmental anomalies. A small number of arachnoid cysts are acquired, such as those occurring in association with neoplasms or those resulting from adhesions occurring in association with leptomeningitis, hemorrhage, or surgery."
There's a start, but it would be best if a specialist doctor could explain exactly what it means to you.