Okay, so "mat salleh" is a term used in Malaysia to refer to people who are from a Western or Caucasian background. It's kind of like calling someone a "white person" or a "foreigner."
So let's say you're at the grocery store and you see a man with light skin and blonde hair. Your mom might say, "Look, there's a mat salleh." That just means he's not from around here and he's probably from a different country like America, Europe, or Australia.
It's important to remember that using the term "mat salleh" isn't necessarily a bad thing, but it's also not a polite thing to call someone to their face. It's always best to use someone's name or nationality instead of a broader term like "mat salleh."