Foremost is an adjective that means holding the highest position or rank. When naming advisors, presidents usually search for the foremost experts on various subjects.
If you are the foremost in your profession, you're the expert people will seek out for an opinion. If your foremost concern in a project is money, all other factors are secondary. You can also use this word as an adverb meaning "before anything else." The film, Gone With the Wind, is set against the backdrop of the American Civil War, but first and foremost, it is a love story.