Issue: Half way round a ride a rattling sound draws attention to loose spokes.
Damage: Not much in some ways. The wheels can easily buckle once spokes come loose, and there’s potential for more damage to the spokes themselves and the tyres if the issue isn’t remedied.
Remedial action: If you’re not carrying a useful multitool, not much, although you could try temporarily screwing the spokes back in by hand. If you are carrying a tool, hopefully it has a spoke key on it, in which case you can screw them back in and tighten them up nicely. Since spokes should not typically loosen themselves this much, it would definitely be worth investigating the rest of them as well, and making sure that you have an even tension round the whole of the wheel.
Ways to avoid: This is arguably one of the consequences of buying a bike from a bike outdoor chain, as opposed to a local bike shop. Big chains will have a checklist to follow when building bikes for the their customers, but checking spoke tension is more than likely not on it. It certainly isn’t for some. Perhaps a particularly conscientious bike technician with a particularly low workload might think to check them, but this will be the exception rather than the rule. So, if you buy a bike from a big chain and ask them to build it for you, check it thoroughly before you ride it. At the very least, ride it first in relatively low-risk places so that any issues come to light before you’re relying on the bike.