DHCP is the protocol that assigns things like IP Configuration, DNS etc to all the devices on a network. Without DHCP you'll have to manually assign static IP addresses to all devices.
When you turn it off, the clients are no longer given IP Addresses. This is needed if you want to use router as an Access Point because you don't want the AP to assign IP Addresses to clients as the main router is meant to do it.
When you turn it off, the clients are no longer given IP Addresses. This is needed if you want to use router as an Access Point because you don't want the AP to assign IP Addresses to clients as the main router is meant to do it.