It’s essentially Affinity Designer for desktop (a rival to Adobe Illustrator) shoved into your iPad and optimised for the touchscreen. In short, it’s all about drawing with vectors.
Vectors remain pin sharp when zoomed in to a ludicrous degree (and Affinity Designer for iPad has bonkers zooming capabilities), hence them often being used for logos and interface design. But many illustrators and artists prefer them, too, because every line or object you draw always remains individually editable.
With Affinity Designer for iPad, you get a full suite of vector tools, raster brushes for textures, symbols, and smart shapes, all rethought for multitouch on-the-go iPad shenanigans.