It's a junction that widens at it's mouth so you could be potentially hidden from traffic coming up behind you by fences, walls or hedges. If you are turning right don't ride right to the side of the flare so you can't be seen but equally don't be so far from the kerb that a car could get between you and the kerb. The BHS Ride Safe Guide should have diagrams, the old Riding and Road Safety manual certainly did.
We were taught in our Riding & Road Safety training that instead of following the flare off to the left you should ride a straight line as if there was no flare. Then in our exam we were all told off for doing that, although we all passed.