What is foci of T2 hyperintensity in the subcortical and periventricular white matter?

This phrase is a statement that would be used by a radiologist when reviewing the results from a MRI. Breaking down the phrase by individual parts:

T2 - An indication of the type of scan that was done. In a T2-weighted scan, areas that are fluid-filled appear bright, while areas that are fatty appear dark. A T1 scan would show the opposite results.

Hyperintensity - An indication of a bright region on the scan.

Foci of T2 Hyperintensity, therefore, means "focal points, or concise areas, of very bright spots."

Subcortical and periventricular white matter - These are locations within the brain. Regions of the brain are categorized by color (white matter or grey matter) and location (cortical, or related to the cortex, subcortical, or below the cortex, etc).

The statement, therefore, means "white spots on a MRI scan at certain locations within the brain."

This statement alone does not indicate any particular disease is present. It is a piece of information that a neurologist would use to help determine whether or not someone had a certain disease or condition. There are many conditions or diseases which can cause white spots on the brain; only a neurologist can sort through the possibilities and determine what caused these white spots.