Lightroom Mobile’s Lens Blur uses AI to detect the foreground and background of your image. You can use a slider to choose the amount of blur to apply, from 0 to 100 in intensity.

For example, the standard kit lens usually has a maximum aperture of f3.5 and ideally you'll what something much wider (lower f-stop number) like a lens that starts at f1.8, f2.0, or f2.8.