The one notrump opening denotes a balanced or semi-balanced hand with 12-14 HCP which can include a singleton or a five-card major suit.
Opening one of a suit is natural but forcing, unlimited in strength that contains at least 14 high-card points (HCP).
The 14-HCP minimum for an opening one bid is only a guideline; a hand with much "playing strength" (for example, a hand rich in aces and kings, long suits or a solid pattern of sequential cards) can be opened one of suit with fewer HCP.
Opening one notrump (1NT) means 12–14 HCP and a balanced or semi-balanced hand, possibly a singleton with three four-cards suits and can include a five-card major.
Opening two in a suit is natural, showing a hand with unbalanced distribution and limited strength.