The DHCP server assigns and provides the IP addresses to the devices in the local network from a pool of IP addresses.
That way, there is no conflict of IP addresses among the devices in the local network.
The lease duration is the duration for which the IP is leased to the device. The address pool begin and end give the range of IP addresses from which the IP addresses are assigned.
Make sure that subnets are clearly understood while creating them as to not create discrepancies. For e.g. 255.255.255.0 subnet will have 253 (given it also has the gateway IP included in the same subnet) usable addresses in it whereas, 255.255.0.0 will have 65,534 usable addresses in it. You can always use this to easen the process.
There should be only one DHCP server running in the LAN (after the IO is setup to be running on your network) so that it doesn't interfere with other DHCP server.
That way you could leave out some IP addresses that you may want to dedicate to some devices statically, like servers.
You can also make sure that a device with a particular MAC address gets a specific IP by mapping that MAC address to that IP in the MAC tab.