what does "seasonal storage"mean?
“We’ll both get drunk and bang on the generator.”
“That’s a plan. I did a kind of half-assed cleaning down there before you came, at least in the main area, the wine cellar, seasonal storage. I don’t really go beyond there, and I don’t think Hester has in years. The rest of the place is huge and dark, dank and just pretty scary,” she told him as they started downstairs. “It’s not my favorite place.”

Question

what does "seasonal storage"mean?
“We’ll both get drunk and bang on the generator.” 
“That’s a plan. I did a kind of half-assed cleaning down there before you came, at least in the main area, the wine cellar, seasonal storage. I don’t really go beyond there, and I don’t think Hester has in years. The rest of the place is huge and dark, dank and just pretty scary,” she told him as they started downstairs. “It’s not my favorite place.”

Answer

It's where they'd keep winter items like heavy coats in summer, and summery things in winter.

Answer

Seasonal storage is the area where you store the items that you only use during certain seasons. For example, if it's winter now, you might store boxes of summer clothes, your gardening tools, your bike. During the summer you move that stuff where it's more easily accessible and store your winter clothes, snow shovel, skis,ice skates, etc.