Convert a hostname/domain into an IP address, resolved from this server rather than from your own device. If a host resolves fine here but fails on your phone, the problem is almost always your device's DNS, not the host itself.
Resolves a hostname to the address it currently points at, from this server rather than from your device. That difference is the point: if a host resolves here but not on your phone, the problem is your DNS, not the host.