What's meant by flats?

Hi girls. New member to the girl world here. I'm going to a friend's birthday party soon and I asked about a dress code. She just said "a cute skirt and flats would be fine". While I appreciate that she's treating me like a girl (m2f here). I have to idea what she means by flats. It doesn't help that I'm a size 15 women's shoe, my options are limited!

So what does she mean? Wtf are flats? Like ballerina flats? I don't want to wear them all night.....

It's a few weeks away. I can still buy stuff. Pics and recommendations welcome.

I'm in Australia, Melbourne if that's at all relevant.....

>>2
I don't think flats are a programming language.