Al dente pasta simply means to cook pasta slightly under done, which offers a slight resistance when bitten into. Al dente pasta can only be achieved with dried extruded pasta.
Here are 3 degrees of doneness. The one on the left is undercooked. You can see there is a white ring surrounding the still dry pasta in the center. The pasta in the middle is cooked al dente, which is what we are looking for. The center has a solid white dot, indicating that it still has a bit of a bite. The one on the right is slightly overcooked, as you can see the center is fully cooked through and it has started to swell and absorb excess water.