Masu vs Iru

I'm back from my long hiatus. (lol, only like four days.) So I was watching Mira's latest video about yellow watermelons. I've heard about them, but I've never seen them nor tried em before. Have any of you? Anyways, under the comments someone asked about the difference between "masu" and "iru." Their comment was very helpful since Mira kindly explained the difference.

"Masu and iru?? They are completely different. Masu is the polite form of a verb that means do/will do something. For example, I don't eat bread - pan o tabemasen I do eat bread - pan o tabemasu . [pan o] tabete iru means I am eating bread."