There are many ways to define it: as an activist slogan, a social and political movement, an umbrella concept, an academic current...
All in all, it's a call for radical social, political and economic transformation in order to bring so-called 'developed' societies back into balance with the living world, in a way that reduces inequality and improves human well-being.
Degrowth is a pluralistic, subversive and open-ended utopia: a call for a new, qualitatively different world that will emerge from and through confronting the existing one.
Degrowth is not alone. It is just one among many alternatives to development. All these worldviews and practices compose the Pluriverse: "A world where many worlds fit", as the Zapatistas say.
For a more detailed definition, check out the amazing degrowth.info website. Feel free to delve into their many resources, and if the idea speaks to you, welcome in the community!