A wet towel dries when water leaves the cloth and moves into the air. This process happens faster near a window for two main reasons.

First, sunlight warms the towel. Warm water molecules move more easily, so they can leave the cloth faster.

Second, air moves near a window. When fresh air passes the towel, it carries some water vapor away. Then more water can leave the cloth.

A towel in a closed, dark corner dries more slowly. The air around it already has moisture, and the towel stays cooler.

This is why people often hang laundry near a bright window. It is not magic. It is heat, air, and patience working together.