What does it do? The whole point of motor oil is to stop all of the inner metal surfaces of the engine from touching each other and grinding together, causing wear on the parts and extra heat that you don't need.
Other components related to the oil: It's not just the oil in your engine that's keeping everything lubricated. There's also the oil filter. The oil filter keeps the oil clean and free from any contaminants that it might collect while it travels around your engine.
There's a number on my oil can, I don't know what it is...
Chances are the number you're looking for is similar to 5W40. This is the oil's viscocity. Viscocity is how easily the oil will be able to move through your engine. The number to the left of the W is the viscocity of the oil when it's cold. A lower number is preferred for starting engines in cold climates. The number to the right of the W is the viscocity when the oil is warm, like when the engine has been running for awhile.
5W40 is your typical engine oil.
Replacement: It's important to change your oil regularly. At high temperatures, the oil breaks down and doesn't lubricate the engine as well. If the engine is not lubricated properly, it will cause excess heating and will shorten its life span. It might even seize up on you, causing you to spend lots of money on fixing it.
When you replace your oil, you'll need to open the hood of your car and find the oil cap. Once located, remove the dipstick. It's that long rod attached to the cap. Take a towel and wipe off the stick. Put the stick back in the engine. Let it sit for a few seconds, and remove it again. Take a look at where the oil level is on the stick. You want to fill the oil up to the high mark on that stick. You can gauge the amount of oil you need from the distance of your oil level to the high mark on the stick.