What is the octaweb?
The octaweb is the structure at the base of a Falcon 9 that holds its nine engines — eight in a ring around one in the centre.
It spreads the enormous thrust load evenly into the bottom of the tank rather than concentrating it, and it isolates each engine bay from its neighbours.
Nine engines instead of one is a safety choice as much as an engineering one. If one shuts down, the others can burn a little longer and finish the job.
That is not theoretical: CRS-1 lost an engine 79 seconds into flight in 2012 and still delivered its primary payload.
See it happen
Meet a RocketEvery part, namedSource: Falcon 9 Block 5 — specifications