To bake a cake, you sift the flour to get out the lumps. When you sift, you separate out one thing from another.
When you sort through the mail looking for the bills or go through your photos to find that shot of your dog, that’s sifting, too. Detectives sift through piles of evidence when investigating crimes, and you might sift through the hundred applications you get from drummers eager to join your band, to find Ms. Right. When you're at the beach, you can sift sand through your fingers, and you might see big machines that sift the sand to clean it.