That sentence means the heron stays very still and then catches a fish. Here are the little steps:
- Motionless = very still, not moving.
- Ambush = wait quietly and surprise to catch.
- Together: the heron stays still, then quickly catches a fish.
Three simple ways to say it:
- The heron stands very still and then catches a fish.
- The heron doesn't move and surprises a fish to catch it.
- The heron waits very quietly and quickly grabs a fish.