Jacoby 2nt is a  convention commonly used when your partner opens a five card major and you  have an opening hand with 4 trump support.  It is forcing to game.
 
 Open must further describe his hand as follows:
          1 -  bidding a new suit  at the 3 Level shows a singleton or void in that suit
          2 -  bidding 3 of the major agreed upon shows a medium hand with about 17-18 points
          3 -  bidding 3nt shows a balanced hand in the range of 15-16 points
          4 -  jumping  to the 4 level in a new suit shows a good two-suited hand
          5 -  jumping to game shows a minimum opener, denies a short suit and is generally a signoff
    
Jacoby 3nt is a modification of this treatment. Some pairs play both Jacoby 2nt and 3nt.
Therefore, Jacoby 3nt usually shows an opening hand  with 3 card support and allows the opener to place the contract.   Note:    Some partnerships prefer to keep the 2nt bid as an opening hand with  2 card support.