The cleanout is the place to get into the main line so it can be cleaned out.
Spending some effort now to find out will save you time and money in the future when you have a problem.
Here are some other great questions:
You'd be surprised how many people don't know whether their water heater is gas or electric.
Many things are not worth fixing depending on their age.
My philosophy is that you start putting your pennies back once a faucet or appliance gets to be about 5 years old.
Things just aren't built the way they used to be — some things last a lot longer and some a lot shorter, but I like the 5-year rule because when something does finally break, you're ready to have it fixed.
For kitchen and bathroom sink drain maintenance, I recommend filling the sink up with hot water once every week or two and letting it out to flush your drains. Be sure to use hot, hot water because this helps to keep the grease build up at a minimum. Also, follow that with a cup of bleach once a month.
New garbage disposals come with a little wrench that fits in the bottom of the disposal on the underside where you can work the wrench back and forth to work a clog loose. Have your plumber show you the next time he is over for anything.
Also, there is a reset button underneath you can push that will restore power to it if the electricity trips for some reason. This circuit breaker will also trip if the disposal jams, to avoid overheating and keep the motor from burning up.
I can not emphasize enough the importance of being familiar with your plumbing — the location of things, their age, and how they are hooked up. This can save you a lot of time and money, and help the plumber do a better job.
I will always be happy to answer your questions. If you're not quite sure how to explain what you're looking at, send me a picture from your phone to mine.
My number is (405)473-2769. Call anytime.