Particles have energy. They can have a lot of energy and bounce wildly like in gases; have less energy and flow like a liquid; or have even less energy like a solid. If you take enough of the particle's energy away you get to the tiniest or the smallest amount of energy possible. This is a Bose–Einstein condensate. This makes all of the particles exactly the same and instead of bouncing around randomly in all different directions, they all bounce up and down in exactly the same way, forming something called a 'giant matter wave'.