Gus Rodriguez,
Let's assume we have got only the default vlan residing in the switch and your Router/DHCP is connected in the same vlan interface.
In this case, the clients connected to the default vlan will be getting the ip-address from the DHCP server. Since, the clients and the DHCP server are in the same vlan.
In case, if there are multiple vlans created on the switch. And also, your DHCP server is residing on a different subnet/connected to a different vlan interface of the switch.
Then we would need to configure the bootprelay on those vlans for the clients to receive ip-adress from the DHCP server.