|You need more network cards, to configure your firewall in a special way.|
If you have 3 network cards installed, one can be configured for your local network, let's say 192.168.0.0/24, one can be used for DMZ, let's say 10.0.0.x, where you provide services to be accessed from the internet like mail server, web server, ftp server aso, and one the is configured to connect to the internet.
The firewall will be configured, to grant access from the internet to the servers services in the DMZ, but not to your local network. Also access from the local network to the DMZ is mostly not allowed.
In short words, it's used to separate local networks form the internet or internet accessable servers for security reasons.