In computing, a firewall is a network security system that monitors and controls the incoming and outgoing network traffic based on predetermined security rules. A firewall typically establishes a barrier between a trusted internal network and untrusted external network, such as the Internet. Wikipedia

What is the use of a Firewall on a computer?

Ans: There are many uses of a firewall on an operating system. However, its main purpose is to protect the computer from in and outgoing traffic malicious activities.