In Discover
you can find photos and videos that you might like from accounts you don't yet follow. You may also see Suggested People that chose the same topics of interest as you. The type of content you see on the Discover page is mostly controlled by you via your selected topics and their priority that you set.
The topics you select will not work as Filter but only as Sorting with first priority to your interests. You aren't isolated or limited to only the content related to them, it's just that you'll be shown such content first
How to Appear on Discover
To allow your posts to appear on the Discover page, make sure you've added the corresponding Topics when adding the post. You can also edit the post to add topics. Posts for Discover page are selected automatically based on vast criteria. Main of which, is topics. People on the Discover page also follow very similar criteria and your profile might appear in Suggested to the people that follow similar interests, if your profile is set to public.