When the opener bids 1 ♥ or 1 ♠ , the responder knows that the opener has a 5+card major suit and at least 12 HCPs.
If the responder has 4+cards in the SAME MAJOR suit and 13 to 16 points, the partnership has at least an 8-card fit in that major trump suit and at least 25 points. This is enough for a game in that major suit, or in No Trump. Instead of the responder bidding a 4-level game bid in the same suit, the responder uses the artificial Jacoby 2NT Convention. The Jacoby 2 NT bid asks the opener to show its hand features at a lower level in hopes of exploring a slam bid. It also lets the partnership enter a No Trump contract that scores higher than a major suit contract.
Note: For simplicity, beginners should avoid the use of the Jacoby 2 bid. Instead, use the direct 4 ♥ or 4 ♠ bids to game. This is an easier and quicker way to a game contract in that major suit. Once the beginner has more experience and wants to bid more aggressively, the Jacoby 2 NT Response can be added.