To the 40kers out there.

I'm just getting into 40k, and I picked up the 5th edition rulebook and the 4th edition tyranid codex.

The codex keeps mentioning fleet of claw on page 75 of the Core rulebook. However, since I don't have the 4th edition rules, all that's there are ship rules.

If someone could summarize to me what the tyranid ability fleet of claw does, I'd be much obliged.