macOS's firewall feature blocks unwanted network traffic coming into your computer, and stealth mode makes your Mac essentially invisible to hackers snooping for computers to target. They aren't foolproof features, but they will keep most people from finding and attacking your Mac on public networks.

First, you need to make sure your Mac's firewall is enabled:

• Open "System Settings..." from the  Apple menu
• Go to "Network"
• Click "Firewall" and enable it

Next, enable stealth mode:

• Click "Options..."
• Scroll down and enable "Enable stealth mode" setting
• Click OK

"Automatically allow built-in software to receive incoming connections" and "Automatically allow downloaded signed software to receive incoming connections" should already be checked. Those settings let the apps you already have communicate through the firewall without you having to take any extra steps. Leave those checked unless you know what you're doing and plan to manage app network access manually. You should leave "Block all incoming connections" disabled, unless all you're doing is surfing the Web.