If your Mac has a wired connection to the Internet, and there is no Wi-Fi network available, you can use your Mac as a Wi-Fi network to connect other devices like iPhones, iPads, laptops and game devices. By using Internet Sharing in System Preferences, you can create a network and set a name and password to allow access. This comes in handy in situations where you need to connect Wi-Fi-only devices, but no Wi-Fi is available.
It is important to understand that just because you have a great Wi-Fi signal, it doesn't mean that your Internet speed will be fast. Wi-Fi signal only tells you about the connection between your device and the local Wi-Fi-router. But that router's connection to the Internet is what really determines your speed.
If you are out of the house and need to connect your MacBook to the Internet you can use your iPhone's personal hotspot function. This allows you to connect through your iPhone's mobile data connection. This is handy when you'd rather not use public Wi-Fi or none is available.
You can easily set up a guest wi-fi network with an Apple base station. Using a guest network allows you to share you connection with guests to your house without endangering your main network with all of your devices. It appears as it you have two separate wi-fi base stations instead of one. You can change the guest network password frequently without needing to re-enter the password for your own devices that are connected to your main network.
If you have an Airport Extreme or a Time Capsule you can create a separate guest network. This appears as a different wi-fi network than your main one. Users of the guest network cannot see the computers or other devices on the main network. Setting one up in advance is a good idea so you can grant visitors access to your Internet connection without worry.