Uncomplicated Firewall

Uncomplicated Firewall (UFW) is a firewall tool for Linux.  It's the default fw on Ubuntu.  Here's how to interact with it on a basic level-

https://help.ubuntu.com/community/UFW

To install-

sudo apt-get install ufw

To turn UFW on with the default set of rules-

sudo ufw enable

To check the status-

sudo ufw status verbose

To disable ufw-

sudo ufw disable