Perrin Variation

The idea

After 4.Bd3 Nc6 5.Ne2 Nb4 6.Nbc3 Nxd3+ 7.Qxd3 Ne7 8.O-O d6, Black has traded the knight for White's light-squared bishop, the piece that defended e4 and pointed at Black's kingside. Lichess names 4...Nc6 the Perrin Variation; the moves after it follow Wikipedia's line.

Plans for White

Castle and use the center. The queen on d3 defends e4, and with f4 or d5 White can gain more space. White has fewer pieces to attack with, so playing for space and central control makes more sense than a kingside attack.

Plans for Black

Develop with ...g6 and ...Bg7, or ...Ng6 and ...Be7, and castle. With White's light-squared bishop gone, Black's bishop on b7 has no rival, and the light squares around e4 can become Black's.

Watch out for

For Black, time. The knight takes three moves to trade itself for the bishop, so Black must catch up afterwards, especially if White opens the center with d5 or e5.

For White, the light squares. Without the light-squared bishop, e4 and the kingside light squares need defending by the queen and knights.