What does ‘al dente’ mean?

It means ‘to the tooth’ and when applied to pasta it means a slight resistance to the bite, not half cooked, but keeping an interesting texture (over-cooked pasta is mushy and sloppy). Remember pasta continues to cook after draining and saucing. The only way to master this timing is to practise, make a lot of Spaghetti Aglio, Olio, and learn as you cook and eat.