To parboil or blanch is to cook by boiling (typically in a saucepan), but only up to the point where a vegetable, pasta, or other food is beginning to soften. Some refer to it parboiling as “precooking”, meaning that the ingredient will be cooked again by another method, so you don’t want to boil it too long.
However, I often parboil or blanch vegetables to serve them cooled or at room temperature. The parboiling takes the raw “edge” off, but leaves them tasting fresh. For example, you would parboil the vegetables for a composed salad or a Salade Nicoise.
When you parboil or blanch (unless you will continue cooking them immediately by another method), drain the ingredient(s) in cold water to stop the cooking process as soon as it(they) reach the desired texture.
Parboiling pasta is common in recipes for lasagna or other dishes that involve baking after the pasta is cooked. Noodles are typically parboiled in Jewish kugel or noodle pudding too, because like lasagna, that dish is baked after the noodles are boiled.
There are only three exceptions to parboiling pasta or other noodles. The first is if the they are fresh. The second is if they specifically sold as “no boil”, as in my Butternut Squash Lasagna. The third exception is when you let noodles rest in a liquid before baking. That method, which seems like magic to me, allows the noodles to soften (as if they were boiled) before baking. Then they don’t need parboiling. I’ve used that technique in two kugels or noodle puddings. One is sweet and the other savory.
Blanching is a broader term for pre-cooking than parboiling. It includes frying for a few minutes in preparation for another stage of cooking, such as deep frying. Parboiling is a type of blanching.
The technique (to blanch or parboil) is simple. The main requirement is that you watch what you’re cooking carefully, so you do not overcook it.