A bonding system ties a boat's underwater metals together with wire, usually green, run to a bonding strip and on to the anodes and the DC negative. The idea is that one set of anodes protects everything bonded to it.
Whether through-hulls and seacocks should be bonded is a long-running argument. Don Casey, in SAIL, says underwater metal that is or can be isolated shouldn't be, while noting the ABYC standard recommends tying underwater metals together. Nigel Calder, in Good Old Boat, says generally no for bronze in a fibreglass hull, with one exception: bond any fitting that could be part of a stray current path.
What a surveyor checks
What's fitted and whether it's coherent. Follow the wires from the bonding strip to each fitting and check every connection for corrosion and tightness. Note which fittings are bonded and which aren't. Look at the fittings themselves: pink, pitted bronze means the zinc has left the alloy. A half-bonded system with corroded wires is the usual finding.
In the report
"Bonding: engine, shaft and two of five through-hulls bonded, remaining three isolated. Bonding wire to the cockpit drain seacock corroded through at the lug. Decide on one approach, then renew or remove the wiring to match." See through-hull bonding.