A spinning top works by the body of the top rotating smoothly around an axis. The axis is called a rotation axis, or a symmetry axis.
The way the top spins around the rotation axis gives it angular momentum, and the angular momentum will remain constant until an external source of torque acts on the spinning top. This means, if a perfectly balanced, which means perfectly upright, spinning top achieves angular momentum after a source of torque makes it start spinning, the top will keep spinning until an outside source affects it, such as gravity or someone touching it.
The pressure applied to the top to start the spinning motion is called torque. Torque can be applied by a snap-twisting motion of fingers, or by pulling a cord wound around the neck of the top. Some toys have a spinning gear that is activated by pulling a plastic, toothed strip through it to launch the top.
When a spinning top starts to wobble and then falls over, that is because torque starts to pull on the bottom of the top as its spinning slows. Because the spin axis must fall sideways, the torque makes the top fall sideways rather than straight down.