A Radial mask only affects a circular area of your photo. It's especially great for making subjects stand out against the rest of an image. In Polarr, you'll find the Radial mask with all of our other masks in the local adjustments panel. Select it to try it out.
Once you've selected a Radial mask, you can pinch to resize or drag to move and rotate. Use the feather slider to change the fuzziness of the mask. Or select invert to switch the affected area of the Radial mask.