DHCP requests sent by the client device, or client computer are broadcasts. Routers do not pass broadcasts through to other networks. This prevents a broadcast problem on one network from propagating to another. To use DHCP across a routed network, a DHCP/ BOOTP relay agent must be employed (RFC 1542). ( DHCP is an extension of the bootstrap protocol (BOOTP), refer to RFC 951).
A relay agent is a program used to pass specific types of IP packets between subnets. A DHCP/BOOTP relay agent simply passes DHCP/BOOTP messages (IP packets) from one subnet to another. It can be software or hardware based. Some routers are RFC 1542-compliant. These have an agent built in to them.