The "Network" section in a virtual machine's Settings window allows you to configure how VirtualBox presents virtual network cards to your VM, and how they operate.
When you first create a virtual machine, VirtualBox by default enables one virtual network card and selects the "Network Address Translation" (NAT) mode for it. This way the guest can connect to the outside world using the host's networking and the outside world can connect to services on the guest which you choose to make visible outside of the virtual machine.
This default setup is good for probably 95% of VirtualBox users. However, VirtualBox is extremely flexible in how it can virtualize networking. It supports many virtual network cards per virtual machine, the first four of which can be configured in detail in the Manager window. Additional network cards can be configured on the command line with VBoxManage.
Because of the vast array of options available, we have dedicated an entire chapter of this manual to discussing networking configuration; please see Chapter 6, Virtual networking.