Rashi: He speaks flattery.

Malbim: "Al" is like Avel (sin). If one flatters his colleague's sin with smooth talk, and says that it is not sin, he sets a trap for his feet. He wants him to increase sin until he will be caught in the net. He is like one who is Machalik (smoothes) the path in which the Chayah goes, so it will be trapped in his net. So he smoothes the crests of the evil ways of his Nefesh, so he will be trapped in the net of sin, and be cut off.