An army of other people's devices - routers, cameras, computers - assembled without their owners' knowledge. This is the crowd that floods are made of.
The most uncomfortable part: the soldier in that army might be your router. A device with a factory password and skipped updates gets taken in seconds by an automated scanner that sweeps the entire internet around the clock. Nobody chose you. Your door just happened to be unlocked.
The infected device keeps working normally. The camera keeps filming, the router keeps serving internet. That is why nobody notices: the army stays quiet until the operator calls it up.
What depends on you
Three things, all boring: change the factory password, run the updates, retire the device its maker stopped supporting. You are not only protecting yourself. Every cleaned device is one soldier fewer in someone else's army.