What is a "firewall" and do I really need one?
The simple answer is YES you need one but why is another matter to explain....
This is a really simple and a basic explanation but here goes.
A computer is like a "house". Every "house" has an address. You can walk down a street and knock on a door to see if anyone is home. If no one answers you can turn the door to see if it is open and let yourself in.
Every computer on the Internet has an "address" a firewall tries to hide this address from the outside world. If someone does not see the "address" chances are they are not going to knock on the front door to see if anyone is home. A "firewall" also locks the front door so that they just can't let themselves in.
A hardware firewall such as a router works much the same way as a "Hotel lobby". Your Internet connection connects to your router and all your computers plug into the router. The router becomes a "Hotel lobby" and all the other computers become protected "rooms". An outsider can walk into the "Hotel lobby" but would not be permitted to see/view or visit any of the "rooms".