Page goals will register if Audience has been true once that session and the visitor visits an URL that matches the value entered in the goal.
Click Add new goal. Then enter a name for your goal and then choose Page goal in the dropdown. Then you need to specify your target URL and choose the correct match method for this. For more information about matching methods:
How to match with exact match, simple match, contains & regex
Example
An very ordinary goal (at least for e-commerce) is to see how many visitors that acutally goes to the checkout page. This could easily be set up (depending on your checkout URL) by adding this as a Page goal using Contains as match method.
This goal setup will register a conversion if the customer visits a page with an URL that contains /checkout. This setting however assumes that there isn't a way for the customer to go to a sub page of /checkout without first passing the actual checkout page.
If you set a Page goal where you think the visitor can actually reach a sub page of the entered URL without actually visiting the entered URL then you might want to use a more strict match method. That is if you only want to track that entered page and not it's sub pages.