What does encroachment of the ventral subarachnoid space in the L4-L5 level mean?

• Encroachment: Taking up space to which it's not entitled.
• ventral: belly side
• subarachnoid space: the area just under the membranes surrounding the spine
• L4-L5: the space between the fourth and fifth lumbar vertebrae.

So, to sum up, something (probably a herniated disk?) is taking up space on the belly side of the spinal cord between L4 and L5. It's not pressing the cord, but it is crowding the space between the spinal cord and the surrounding membrane.