Strange matter is a kind of stuff that is made up of tiny particles called quarks. Just like how atoms make up everything around us, quarks make up the stuff that scientists call "hadrons." But strange matter is special because it has one very special quark called the "strange quark."
Normally, quarks stick together in groups of two or three to make up bigger particles like protons and neutrons. But in strange matter, there are lots of strange quarks all stuck together, without any other quarks around. This makes strange matter really weird and different from anything else we know about.
Scientists think that strange matter might be really heavy and stable, and some even say that it could be dangerous if it ever got loose! But don't worry, because so far nobody's ever found any strange matter on Earth.