Psalm 119:66 (net) Teach me proper discernment and understanding! For I consider your commands to be reliable.
Discernment here, is that by which we determine the moral quality of things, or decide what is right or wrong; wise or foolish; good or evil (Barnes).
If we can’t discern between good and evil and truth or lie then we will always succumb to the lie (2 Tim 4:3-4, Heb 13:9).
This is especially true in the church. There are a lot of false teachers who are perverting the Gospel (Jer 29:8, Matt 24:4, Rom 16:17, Col 2:8, 2 Peter 2:1-2, 2:19).
So, to avoid being led astray, we must learn discernment.