Brew in a bag method is taking a big mesh bag and filling it with your mashing grains.. then you "steep" it like a giant tea bag in the strike water to mash it... It works, I use it for oatmeal stouts sometimes when I need the convenience of lifting a bag out to remove the grains rather than hassling with lautering. The pros is convenience and speeding up brew day.
The cons to this method is efficiency, you're going to maybe have around 40-60% tops efficiency in extracting all the sugars the grains offer, so if you are to mash 2lbs of something but want to brew in a bag, you'll want to up your 2lbs to something like 3 or 4 to make up for the lost efficiency.
If you can get a big enough bag that lets all the grains float freely and you break up all the doughballs you could get around 70% efficiency and maybe a little more depending on how well you rinse out the bag with sparge water.
Cheers!