There are details, and even a couple of videos, here:
Basically it works on the principle that we mostly go through our days looking at the negatives in our lives, or at very least not really focussing heavily on the positives.
This is such a down way of going through life, that it can actually lead to mood problems, depression, etc.
One lady decided that to make herself focus on the positives in her life, she would take one photo a day, for a whole year, of something that she is grateful for.
The project made her think all day about the next subject for the day's photo - "What am I grateful for?" "What thing that I'm grateful for, could I take a photo of today?"
It transformed her life: She had thought that her husband was unromantic and it was affecting their marriage, but looking for things she's grateful for, she started seeing the little positive things he does each day. She changed her focus from negatives, to positives, and it transformed her attitude to her husband, and that transformed their marriage even further.
She had been battling depression, and this project became a natural therapy for depression that worked wonders.
As she told people about it, and as they then followed suit, they also started to see changes in their lives.
I was recently doing a Photo A Day challenge, and found myself walking around all day looking for the day's subject. Walking around with half my mind wondering what will I take a photo of that is a "Goal", or how will I photograph today's topic, "Colour", or how can I make a unique twist on a photo of the subject, "Letter"?
I foresaw very quickly that by the end of the year, all I'd have is 365 photos of 365 different subjects, and that a lot of my attention each day was going to be on pointless topics, rather than on something that might actually improve my life and my outlook.
I started doing this project myself on the first of February, so have only 6 photos so far. But I can already see how having half your brain tasked with "What am I grateful for, that I can take a photo of today?" is going to be quite different to having half my brain tasked with finding, a "Crazy" photo, or a "Treasure" photo, or some other random subject off a list.
If you're working off a challenge list, please don't take offence. I'm not having a go at these kinds of random topic lists. The lists are great for taking people into areas of photography they might never have tried, or even just making you pick up your camera and use it every single day. This is just another way of doing that, but you have to look to find the subject for yourself each day, and that makes you look at lots of subjects all day, and because they're all things you're grateful for, it starts to change your whole outlook on life as well.