An Opening bid of 3NT shows 16-21, a solid or semi-solid minor suit with at least 6-7 cards. Unlike Gambling 3NT, opener is expected to have stoppers in at least 2 (usually all 3) outside suits. Further, opener should not have void or singleton.
There is a defense to this convention, which is an extension of ASTRO.
Double is for penalty
4 Clubs shows hearts and a minor. Partner bids 4D to run to the minor.
4D shows spades and another suit. Partner bids 4H to run to the other suit.
All other suit bids are natural.