What is a firewall useful for?

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I'm a regular average joe, but I want to become more interested in computer technology. So a firewall is a software that interacts and blocks people from entering a network. What use is this to me as an average non-professional? I heard you can block certain IP addresses too. Is there anyway I can experiment with stuff, since I don't want to mess with my family's main internet connection, and I connect by a Netgear adapter.

It's useful to you for multiple reasons.

Host-based firewalls act to protect a system against unwanted access (generally, they only allow traffic from connections initiated by the local host).

Firewalls on a gateway (like a router), besides host defense, can act to perform traffic shaping, such as NAT (Network Address Translation), and can act as a proxy for another system.

https://en.wikipedia.org/wiki/Firewall_%28computing%29#Types

As you have surmised, a firewall is software designed to control what gets into your computer.

It's largely a one-way filter. A hacker should not be able to get in, but if you go out and load a webpage with malicious code, it will bypass the firewall because YOU initiated the connection. This is why most hackers try to get people to download malware either in e-mail attachments or in scripts hidden on a webpage.

A firewall can work BOTH ways if you want to create "black lists" or "white lists." A black list is a list of banned websites that nobody can view. A white list is a list of approved websites. Both work in similar ways...depending on your need for security.

Most people would use a "black list" to ban access to sites they don't want people going to (e.g., pornography). The problem is that you must always be aware of new sites and update the "black list" to ban new sites.

You would use a "white list" to ban everything EXCEPT pre-approved web sites. A company that uses the Internet for work purposes and doesn't want people using it for anything else would be better served with a white list because anything NOT on the list is automatically blocked.

Your router has a way to log in (an IP address). Be sure to learn how to do a master reset of your router. Normally it involved holding the reset button for X seconds as compared to just pressing it. This means you can fiddle with settings and if you mess up, reset the router to undo whatever you did wrong.

Of course, be sure to know how to reprogram the router because some ISPs have specific items you must enter in to tell the router how to log on to your service provider. In a master reset, you'd have to reenter all of that information.