Google acquired Snapseed close to a year ago, and it looks like the Mountain View-based giant is finally using its acquisition by adding some photo editing tools based off of Snapseed's technology.
You can use these photo editing tools by looking at one of your photos in a lightbox in Google+ and clicking the new "Edit" button above the image which will then display a sidebar with a bunch of options. These tools give you the ability to adjust, crop, rotate, add borders, selectively adjust, tilt-shift, apply several filters and more.
Once you've finished editing your photo, you can either save it or replace the original, if you've made a mistake, you can revert the changes and go back to the original photo. The new Snapseed-like tools should have rolled out to most Google+ users, but if you can't see them yet, give it another couple of days.