It's like a reverse sear, but more precise.
Water boils at 212, right? Medium rare on a steak is 130, rare is 120. If you drop a steak in 212 degree water, it will turn to soggy shoe leather after a while.
Sous vide is a heating element, propeller, and thermometer all working together. If you know you want to cook your steak to 120, you set the temperature accordingly. The heating element heats the water in whatever container you're using (a pot, a plastic bucket, your sink, whatever) to the desired temperature and the propeller moves the water around so it is all a consistent temperature (no hot spots). So you put your steak with some butter and maybe some rosemary with salt and pepper in a plastic bag and drop that in the 120 degree water. After a while, your steak hits the same 120 degree temperature of the water around it. The steak will be 120 degrees all the way through and if you leave it in 10 minutes longer that you should have, it's still only 120 degrees. You take it out of the water and the plastic bag. It's fully cooked, juicy and delicious but there's no crust on it. Eating it like that, it would taste like you took a perfectly cooked steak, skinned the crust off of it, and just ate the meat in the middle. That's no good. Well, it's good, but it's not good enough. You want that nice crust. So you pat the steak dry and give it a quick sear in a screaming hot frying pan or over hot coals. Just long enough create that magical crust because the meat is already cooked through. If you do it correctly, your steak will have a thin crust and consistently cooked meat as opposed to a hard crust followed by a layer of gray, then pink, then red in the middle.
Lots of restaurant food is pre-cooked in giant commercial sous vide machines. When you order it, all they have to do is put a sear on it. It's faster and you don't have to worry about a bad cook screwing it up.
If it is something that doesn't need to be seared, like shrimp or vegetables, it's easier.
Alex.