Targeting means on which page your guides are going to start to show your users. We're setting up the page of your guide's first step as a targeting page by default. You don't need to set it manually when you want to create product tours. However, if you want to show a one-step modal on every page you have (e.g. Happy New Year Off), you need to set up a targeting page manually to show the guide on every page of your website.
Segmentation means what kind of users going to see your guides. Segmentation is related of your users segment, not which page you have guides.