I purchased a MacBook Pro. Inside the box is the power source. One is the square piece that plugs in outlet and other end has USB plug for computer. What is the second cord for? It had 3 prong plug with white plastic piece on other end.

AC wall plug. Read the info on the box your computer came in.

What’s in the box.

klink12 wrote:

I Purchased a MacBook Pro. Inside the box is power source that plugs into wall and USB connection on the other. Also another cord with 3 prong plug on one end and a white plastic piece on the other. What is this second cord for?

That is not a USB plug. Don't try to plug it into a USB port.

You don't have to use the longer cord, but as a 3-prong plug it is a fully grounded extension. I use it because the wall outlet is often farther than the thin cord.

If you slide off the "duck head" module (the 2-prong plug that swings) from the power brick, you will see that reveals a place for you to attach the longer cord.

Apple ships it in two pieces for international compatibility. If you had bought this in another country, you would have gotten a different AC cord with a different country-specific plug on the end, along with a corresponding duck head for that country.

If you buy the international power adapter kit, you can replace the 2-prong duck head plug module with a module that fits the AC plugs in the country you are in.

If you have used Windows PCs in the past, this is not that different from their power supplies, since many of those also come with one cord from computer to power brick and a second cord from power brick to wall. Apple's just looks a little different, but is also usable without the second cord.

This picture shows how the cord is used.