What does al dente mean?
"Al dente" means "to the tooth" in Italian. (Like "terroir", it's one of those concepts that poorly translates into English.) The phrase refers to the desired texture of cooked pasta, which should be soft but still slightly firm at the core of the noodle (or shell or spiral or alphabet letter). Some cooks define "al dente" as "not hard and not soft."
Pasta cooked "al dente" should require some chewing but not crunch or stick to the teeth when chewed. The firm texture should allow you to taste some of the pasta's flavor. Overcooked pasta tends to be mushy and flavorless.
So how do you know when your pasta is "al dente"? In my house growing up, we threw a strand of spaghetti at the wall -- as soon as it sticks, it's done. The problem with that test is that overcooked pasta sticks to the wall, too. So now I just use my teeth: Before I think it's ready, I draw a piece of pasta from the pot, let it cool a few seconds and take a bite. If it's ready, my mouth knows.