This convention is used after 1maj-2maj to show an invitational hand (or Slam interest). New suit at lowest level ask the responder for help in the suit. The opener has 3+ card with at least 2 losers. The responses are:
3maj: minimum and not much help.
3 in new suit belowe 3maj: Not help in the trialbid suit, but maksimum hand with honours in the bid suit.
4maj: A maximum hand with some help, or excellent help in the bid suit.
Help in trialbid suit is top honours or short suit. But with short suit help the responder should have extra trump.
Since trialbid is an invitational bid, 1maj-2maj-3maj show a weak hand and is pre-emptial.
Some also use trialbid to show a short suit. Responder should then look at honours in the bid suit as negative, and help in the other 2 sidesuits as positive