What does it mean to incorporate in recipes

Fix Compacted Flour. Flour will compact over time (and during shipment). You could sift the flour to fluff it back up. Or, you could just stir it before measuring and be sure to spoon the flour into your measuring cup in order to get a correct volume measurement.

Remove Unwanted Material. Yes, sifting would also remove larger pieces or bits of chaff. It would also remove insects. However, neither of these are problems with modern store-bought flour. If you grind your own flour (my sister-in-law does this), then you may still want to sift it, though.

Mix Ingredients Together. Sifting can also be used to mix other dry ingredients into flour. I bake quite a bit, though, and I've never had a problem with just using a spoon or whisk to mix dry ingredients together.

In summary: don't bother. Just use good measuring techniques and stir your ingredients together well. No one likes a lump of baking powder lurking in a muffin!

I've never heard of any two things which are safe to eat separately, but poisonous when combined. When you consider everything gets mixed in the stomach anyway, I find it unlikely to ever find things like that.

Daniel's question about your food safety knowledge is an important one. Make sure to use separate tools for raw meats, clean your tools, cook to appropriate temperatures, and not leave food out either thawing or cooling. I wouldn't feel bad if -everyone- took a food safety class at least once in their life. I mean, it's typically a few hour workshop.

In the long run, I would encourage you to aim for a diverse diet. Make sure to include starches, green and root vegetables, and other oddities. The underlying attitude of "I'll eat whatever is quickest to throw in a pot" can lead to frequently finding the same thing "quick to throw in a pot" and a less healthy diet.