Firewall

Short answer:
A firewall is a piece of hardware and/or software which sits between your PC or network (the inside) and everyone else (the outside). All transmissions between the inside and the outside flow through the firewall. The firewall checks each transmission against a list of rules, which you define, and either allows or blocks the transmission based on information in its header.

For more complete answer:
If you search the Techrepublic site with the keyword 'firewall' you will finda large number of lengthy and very good descriptions.

You want a firewall because without it you are vulnerable to anyone on the internet who wants to reach out and touch your computer, which may include, but not be limited to, changing files, deleting files, or using your computer to attack someone else.

The disadvantage of a firewall is the cost (a really good one for an enterprise can run thousands of dollars), and the amount of time it takes to set up and keep it running. This cost isusually a tiny fraction of the cost of not having a firewall and of having someone break into your network.