Apologies in advance if I'm missing some context here but from my understanding it's pretty straight-forward: in the U.S. an individual is insured by the FDIC up to $250K. So anything you have over that in the one account is "susceptible" to bank failure losses (and even then it's not guaranteed that you'd lose your $...just possible).

So if you have $1M parked in one account then you're at risk. $250K parked in 4 accounts caries zero risk.