3NT
This if its bid in the opening position means the opener has 25+ points and a balanced hand.
As a sign off bid it means game in No Trumps.
The bid means you have contracted to win at least 9 tricks with no trump suit.
The leading or first card played determines the suit; and then the highest card played in that suit takes the trick
The suit distribution of the hand can be in either of the three following patterns
5, 3, 3, 2 In any order
or
4, 3, 3, 3 In any order
or
4, 4, 3, 2 In any suit order