Yes — the sentence is grammatically correct.
Why it is correct (simple steps):
- Subject: "The dump truck" — that is what the sentence is about.
- Verb: "lifts" — it tells what the dump truck does.
- Object: "its bed" — this is what the truck lifts.
- Purpose: "to pour out sand, gravel, or dirt" — this explains why the truck lifts its bed.
Short child-friendly meaning: The truck tips its back so the sand slides out.
Small notes and alternatives:
- "Pour out" is correct, but another common and simple word is "dump": "The dump truck lifts its bed to dump sand, gravel, or dirt."
- You can also say: "A dump truck raises its bed and dumps sand, gravel, or dirt."
- The comma before "or" (the Oxford comma) is optional here: both "sand, gravel, or dirt" and "sand, gravel or dirt" are fine.
All versions are easy to understand and grammatically correct.