This is a makeshift creep feeder, a dry place to put a small trough of lamb food, with bars across the front (well, back if you are looking at this photo - the bars are out of sight) so that the ewes can't get in but the wee ones can.
Not that they are wee any more...
The creep feed allows the few smaller lambs to supplement their diet if they aren't getting enough milk. We don't supplement the lamb's diet unless a ewe or two is struggling to keep up with the demands of her young, preferring a slow grown grass fed lamb, so the creep feed won't be made available for much longer.
It's crucial to remember (and that doesn't always happen) NOT to put the ark too close to the fence, or with a couple of hops, skips and jumps the lambs bound over the fence and into the blue yonder, hysterical with freedom until they are utterly unable to get back to mother and bleat piteously for some human sap to come and sort it all out.
Once the first lamb is up on the ark, rattling the tin with their sharp percussive hooves, their mates join in until there is no more room at the inn.
Oh, and The Last Ewe (uppercase, up the duff and unpopular) still hasn't lambed.