After a major suit has been raised to the 2-level, the bid of a new suit is a Long Suit trial bid (LST). It shows 3+ low cards in the suit and asks for Partner's help in that suit.
The new suit bid must be below the 3-level of the agreed trump suit.

For example :   1♥ - 2♥ - 2♠   or   1♣ - 1♠ - 2♠ - 3♦

The purpose of the LST is to reach a Gam contract in the agreed major trump suit.

Replies to the LST are :

with 0 or 1 loser in the Trial suit
or with max pts and 2 losers in the Trial suit → bid Game

with 3 loser in the Trial suit
or with min pts and 2 losers in the Trial suit → bid 3 in the trump suit

For example after 1♥ - 2♥ - 2♠ - ?   Responder's next bid (with 6-10 pts)

bid 4♥   with :   ♠ 4 3   ♥ Q 10 5   ♦ A Q 8 2   ♣ J 7 4 3   (10 pts, 2 ♠ losers)

bid 4♥   with :   ♠ 3      ♥ Q 10 5   ♦ A 9 8 2   ♣ J 9 7 4 3   (now 10 pts, 1 ♠ loser)

bid 3♥   with :   ♠ 9 4 3   ♥ Q 10 5   ♦ A Q 8 2   ♣ J 7 5   (9 pts, 3 ♠ losers)

bid 3♥   with :   ♠ 9 4      ♥ Q 10 5   ♦ A 9 8 2   ♣ 9 7 5 3   (7 pts, 2 ♠ losers)

And after :   1♣ - 1♠ - 2♠ - 3♦ -  ?   Opener's next bid (with 12-15 pts)

bid 4♠   with :   ♠ K Q 8 2   ♥ K Q 7 5   ♦ 7 6   ♣ A 9 7 (15 pts, 2 ♦ losers)

bid 4♠   with :   ♠ K Q 8 2   ♥ K 9 7 5    ♦ 7     ♣ A 9 7 3   (now 15 pts, 1 ♦ loser)

bid 3♠   with :   ♠ K Q 8 2   ♥ K Q 7 5   ♦ 7 6 3   ♣ A 7   (15 pts, 3 ♦ losers)

bid 3♠   with :   ♠ K Q 8 2   ♥ K 9 7 5    ♦ 7 6      ♣ A 7 3   (13 pts, 2 ♦ losers)