"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.
Related: What's the point of putting oil in my pasta water?
Related: How to cook garlic for pasta
What do we mean by shimmering oil?
What do we mean by shimmering oil?
Shimmering oil is hot oil that is nearing its smoke point.
At room temperature, common cooking oils like vegetable and olive oil seem fairly thick. Put them in a pan and heat them though, and they thin out when you swirl the pan. As they get hotter, they tend to "flow" and coat the pan more easily.
In the right light, when you look at oil that's at a good temperature for sautéing -- nice and hot, but not yet smoking -- it shimmers. It forms "tines" like those on a wine glass. It looks colorful, iridescent even.
Shimmering oil is good for sautéing because it increases the chances that the food won't stick. Hot oil immediately seals the bottom of food, creating a natural barrier between it and the bottom of the pan.
