Turn on the macOS firewall
A firewall controls which connections are allowed to reach your Mac from the network. macOS has a good one built in, and turning it on adds a useful layer, especially on shared or public networks.
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Switch it on
Go to System Settings, then Network, then Firewall, and turn it on. That's enough for most people, it quietly blocks unsolicited incoming connections while letting your normal apps work as usual.
Optional: stealth mode
Click Options and you can enable Stealth Mode, which makes your Mac ignore uninvited probes from the network rather than replying to them. It's a sensible extra on public Wi-Fi and does nothing to interrupt your everyday use.
Let apps ask
Leave the firewall to prompt you the first time a new app wants to accept incoming connections. If an app you don't recognise asks, say no, you can always allow it later if it turns out you need it.
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