Wide IP-level load balancing

From the Study Guide DNS 302:

"A wide IP contains two or more pools. The GTM load balances requests, first to a specific pool, and then to a specific virtual server in the selected pool. If the preferred, alternate, and fallback load balancing methods that are configured for the pool or virtual server fail, then the requests fail, or the system falls back to DNS."

Do you know what "system falls back to DNS" means ?

Thanks,