"Plain" is used with different food products to mean that there's no extra flavor added to it. Products that you can buy "plain" include:
- plain oatmeal
- plain rice
- plain M&Ms (I don't know if M&Ms are available everywhere, but it's a chocolate candy with a hard shell on the outside. The "plain" ones don't have peanuts inside.
You should also learn the phrase "eat (something) plain", which means that you eat a simple type of food without adding anything to it:
I don't put butter or anything on my toast. I just eat it plain.