Ok, so imagine your DHCP server is on a different VLAN to your AP. Your AP is in one domain, the DHCP server in another.
To use DHCP Snooping, you would configure an IP Helper Address on the AP's VLAN Interface, your AP's Switch port needs to be a member. This basically passes on DHCP request packets to the IP Address you have specified, i.e to your DHCP Server, thereby beginning your handshake process. Inter-VLAN Routing must also be enabled, either in the Switch itself, or via some other means like a router on a stick type configuration, in order for this to happen, the VLAN's cannot be unreachable.
I'll try and give a quick Cisco example off the top of my head for the switch. From terminal config mode (conf t from an elevated prompt):
int VLAN 10
desc Wireless VLAN
ip Address 192.168.1.1 255.255.255.0
ip helper- address 192.168.10.254 255.255.255.0
exit
Commands are basically:
get into VLAN interface config
Add a friendly description for the VLAN
the VLAN ID
The IP address of the VLAN
Your IP Helper - IP address of your DHCP Server
Exit back to Terminal mode
int g0/1
desc AP's switch port
sw mode access
sw access VLAN10
spanning-tree portfast
exit
Commands are basically:
get into Port interface config
Add a friendly description for the Port Interface
Switch Port Interface Number
Set the Port type as an Access Port
Set the Port as a member of VLAN 10
Tell the Port to skip the normal state processing and go straight to forwarding packets
Exit back to Terminal mode
The next part is just a repeat of the first part, but this time for the VLAN (domain) that the DHCP Server is a part of
int VLAN 20
desc Servers VLAN
ip address 192.168.10.1
exit
int g0/2
desc Servers VLAN (The DHCP Server lives at 192.168.10.254)
sw mode access
sw access VLAN20
spanning-tree portfast
exit
Your DHCP Server will, among other things such as DHCP Server Address, Default Router, NTP Server and etc, provide a Default Gateway Address to clients. In the case of devices in VLAN 10 above, your wireless domain, this will be the VLAN Interface IP Address, 192.168.1.1. All clients on this VLAN will use that address to send packets out of the wireless domain that is VLAN 10.
VLAN 20 is where your DHCP is located, different domain, and Switch Port g0/2 is a member of that.
Hope that helps a little
Added a bit more clarity I hope