In physics, "spatial symmetry" refers to the way a feature stays the same no matter which way you observe it. If you were to walk all the way around a sphere, it would look the same at each point in your journey. That's because it has continuous spatial symmetry. A cube, on the other hand, would look slightly different as you passed from one face to the next, but would look identical at each face. This means it "breaks" continuous spatial symmetry and instead has discrete spatial symmetry: you can only see the same thing from specific directions. That's the essence of a crystal: it breaks continuous spatial symmetry.
Symmetry also applies to laws of physics like gravity (you'd see an apple fall the same way no matter how you were watching it) and, importantly, time. The gears on a clock, for example, move continuously at any given rate as they spin on an axis of rotation, so they have a kind of continuous temporal symmetry. Just as a crystal breaks continuous spatial symmetry, a time crystal would break continuous temporal symmetry: its "gears" spin on an axis, but only with specific rates of rotation.
The University of Maryland scientists successfully created a time crystal by using very low temperatures, a magnetic field, and lasers to trap a ring of positively-charged ions. "If this combination is put together just right, then this ion ring will enter its lowest energy state, also known as a ground state," physicist Matt Lowry tells Curiosity. "It ends up that this ion ring time crystal actually rotates while in its ground state." A system in its ground state shouldn't be able to move, but time crystals do—in that symmetry-breaking way. Moving in a ground state means that the crystal could rotate forever without heating up or requiring additional energy—something the researchers say is an entirely "new phase of matter." Learn more about the mind-boggling world of quantum mechanics in the videos below.