When you have a fit (8 cards) in a major suit and opener has a strong hand but a three card weak suit, trial bidding is a way of finding out to what degree the responder can cover the weak suit.
If opener bids 1S/H and responder bids 2 of the same suit then a fit is agreed.
The opener with a strong hand bids any suit with 3 losers (a trial bid) to ask whether partner can cover tricks in that suit.
If responder has 1 loser she bids 4 of the agreed suit. If 2 or more losers she bids 3 of the agreed suit.
If you are using trial bids then once a suit is agreed the rebid in a new suit is always showing a weak suit.
i.e. 1S pass 2S pass 3C (weak suit) pass - 4S (if one loser in clubs) 3S ( if two or more losers in clubs).