To use MitID, you must have a MitID authenticator. You use your authenticator every time you transfer money using your online bank, check your taxes, shop online, etc.
You use your MitID authenticator, when you need to confirm your identity, that is when you log on or approve using MitID. The most convenient authenticator is the MitID app on your smartphone or your tablet. You can choose another authenticator, like the MitID Code Display if you are hesitant about using the app.