A nature-centred religion
Many Wiccans describe the natural world as sacred in itself rather than as a symbol of something else. Seasonal festivals, moon observances and outdoor ritual all follow from that.
Not the same as being outdoorsy
Nature observation happens wherever you already are. City pigeons, weeds in pavement cracks and the changing angle of light through a window are all the real thing.
Where the idea came from
The strong nature-reverence in modern Wicca owes a great deal to nineteenth-century Romanticism and to the twentieth-century environmental movement, as well as to earlier folk custom. Historians generally read it as a modern religious response to industrial life.
Wicca treats nature as the thing itself, not a backdrop, and 'nature' includes the bit outside your window.