What exactly does a firewall do?

I'm not a high-tech person, so I need a simple explanation. What does turning on the firewall on a Mac do?

Also, what does stealth mode do?

Answer: not much.

A firewall protects you from denial-of-service ping-flood attacks. Your router already has a hardware firewall built in, most likely, and that's all you need. There's no need to turn on the Mac's software firewall in most circumstances. "Stealth mode" refers to the Mac's default state online: no unnecessary ports open, or returning results from inquiries.

You do, however, may wish to turn on your firewall when using public WiFi. (Starbucks, public library, etc.)

chscag said:

You do, however, may wish to turn on your firewall when using public WiFi. (Starbucks, public library, etc.)

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This should be reiterated. Hardware based firewalls (such as those found in routers) are great if you never leave the network that is protected by said firewall. If you have a portable Mac that you use on a public or unsecured network, you may want to consider enabling it.

You may want to CONSIDER enabling it, but most public Wi-Fi routers ALSO have a hardware firewall on their routers (since most routers have them, period), so again you're probably just opening yourself up to access issues by invoking the software firewall.

The software firewall is there primarily for the benefit of people who are on routers (or dial-up) with NO hardware firewall. If you know you're in that situation, then by all means you should use it.

The problem is that you don't know which public WiFi is behind a NAT enabled router. It's better to turn on your firewall and be safe when using public WiFi. It's even better when traveling and using public WiFi to have a VPN. (Virtual Private Network)

If you still need further explanation of what a firewall is I feel that this article gives a pretty decent description: Firewall Explained.

Thanks!

chscag said:

The problem is that you don't know which public WiFi is behind a NAT enabled router. It's better to turn on your firewall and be safe when using public WiFi. It's even better when traveling and using public WiFi to have a VPN. (Virtual Private Network)

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Fair enough, though I would add that if you're just doing silly casual stuff (nothing sensitive, financial, medical etc) it may not be worth the hassle to turn all that stuff on.