String pull bends the neck forward. The truss rod pushes back. The balance between them sets how much relief the neck has, which affects action and buzz along the neck.
When it's worth checking:
- After changing to a different gauge or tuning
- When the seasons change and the neck moves
- When the action in the middle of the neck feels higher or lower than it did
Basic guidelines if you adjust it yourself:
- Check relief first so you know which way you need to go.
- Use the correct tool for your guitar. A wrong-size tool can strip the nut.
- Turn in small amounts, no more than about an eighth to a quarter turn at a time.
- Retune, give the neck time to settle, and check relief again.
- Stop if the rod feels very stiff or won't turn. Don't force it.
On most guitars, viewed from the adjustment end, clockwise tightens (less relief) and counterclockwise loosens (more relief). Some necks work differently, and a stuck or maxed-out rod can be damaged by forcing it. If you're unsure, or if the neck needs a large change, a qualified technician is the right call.