The movie follows potential guide dogs from birth through "graduation," when they're given to the people who need them. SPOILER: not all of them make make the cut. If you don't want to read my whole review (which isn't terribly long), Rotten Tomatoes summary is very accurate: "'Pick of the Litter' has all the fluffy adorableness audiences expect from a puppy documentary, along with a story that's as edifying as it is heartwarming."
The directors worked with a charity called "Guide Dogs for the Blind," who train the dogs. They filmed a litter of five puppies, all with "P" names (Poppet, Potomac, Phil, Patriot, and Primrose) as they're sent out to their first homes for socialization and initial training, through their ten week training back at Guide Dog headquarters, and finally to the people they'll spend their lives with. It's staggeringly cute and also quite fascinating. I had always assumed a lot of training went into these dogs: it does, and it's interesting to see what that is. One of the most interesting documentaries I've seen in quite a while.