It's an icon of a lock. There is a circle to represent the lock mechanism, with a small black circle with a rectangle extending from the bottom to represent the keyhole. A curved line extends from the top left of the large circle, and arches almost all the way to the right, to represent an open lock.
Related to Unlock Icon
Related icons are the icons with matching tags, as well as all security icons. All icons are in the same flat style. You can either download free PNG icons or buy SVG vectors