Talkin' 'bout the triad.
It's no secret that the triad - a 3-note structure comprised of a pair of Major and/ or minor 3rd intervals - forms the cornerstone of functional Western harmony.
Even though this might seem like an elementary concept for some, a full understanding and command of the use of triads - along with their inversions and permutations - are a must for any competent improviser, and as mentioned previously, it's one of the basics deserving of a periodic refresher.