The Wikipedia page expresses it fairly clearly in the intro paragraph:

Solarpunk is a literary, artistic, and social movement, closely related to the hopepunk movement, that envisions and works toward actualizing a sustainable future interconnected with nature and community. The “solar” represents solar energy as a renewable energy source and an optimistic vision of the future that rejects climate doomerism, while the “punk” refers to do it yourself and the countercultural, post-capitalist, and sometimes decolonial aspects of creating such a future.