You can do a bunch of things - colourise, add text, add effects, use filters, fix blemishes, round corners - all very useful stuff. I've been a little OTT with the "before and after" below, but you'll get the idea. This is super-easy to use and uses "real" language to explain the effects.
So if you're not a graphic design pro but need to make occasional image edits - personally or for marketing activities like social media- it's a great solution.
And as it's free it's a tool to test drive and then bookmark in your zero budget marketing toolkit.
Off to have more of a tinker....