As part of the coming of age ceremony, the Hamer woman demand that their men whip them. In a society that still fight wars with other tribes over cattle and where men do not sleep in the huts in case of attack, the theory is that the whipping increases the bond between male and female members of the tribe. The wounds look terrible, and must be extremly painful, but cause no permanent damage. There a difficult topics raised by this photograph - please pause before you comment.
As a CC photo, it's not really clear what's going on. If you had wanted to show the woman being scourged with a reed/rod, that would have been the shot instead. And all that behind her is out of focus, at any rate. Interesting story though. But on a continent where the AK47 is king, it seems a somewhat ineffective custom.