The idea is to salt the water in the cooking pot just as it comes to a boil. It dissolves into the water and then pasta absorbs the salt along with the water as it cooks. You're salting from the inside out.
If instead you cook the pasta in plain water and wait to salt until afterwards, the pasta will taste bland no matter how delicious your sauce is that you pour on top.
For a 4-quart pot, I typically add a good tablespoon or two of salt to the water. This sounds like a lot, but only a fraction of the actual salt will be absorbed into the pasta. You need to get the salt concentrated enough so that the pasta absorbs the correct amount, which means adding more salt than might seem necessary. One of my chef instructors always said that you want pasta water, "as salty as the sea" for proper cooking.
Give this method a try and see if you notice the difference!
(Images: Pasta with Asparagus, Pancetta and Lemon Zest/Emma Christensen)