I want to use a separate microphone instead of the one in my MacBook, is that possible? How would it work?
Uke Ukulele Tuner uses the system settings for audio input and output. You can change those in System Preferences, in the 'Sound' section. If you have a USB microphone you'll be able to select it as an input. If you have an analogue microphone you will need an audio interface you can plug your microphone in to. In that case the device will show up in System Preferences, but only the first input of the audio interface will be used. We will probably add settings that allow you to use a different input/output to the system setting in a future update, and add support for choosing different inputs on devices with more than one.
So the easiest route is a USB microphone, but an audio interface is great if you also want to be able to plug your electric ukulele in too. For an example of an audio interface, this is a commonly used model (no affiliation):