This is as simple and lamens term as I can put it...
Network Address :
Your ISP divides each client into seperate subnets. In this case the subnet has 8 ip addresses which means a 255.255.255.248 or /29 network mask. You need 1 ip address to define this network. Minus 1 ip address.
Broadcast Address :
Every subnet needs a broadcast address. This is the ip address used to broadcast to all ip's in your subnet. Minus 1 ip address.
Default Gateway :
You need an ip address on your subnet that knows how to route traffic to destinations outside of your subnet. The default gaetway needs to be assigned an ip address in your subnet. Minus 1 ip address.