If the Host of your Mighty Network has enabled Topics, you'll be prompted to follow some Topics when you first join. When you follow a Topic, your main activity feed will be personalized so that you'll see more of the posts in that Topic. You'll also automatically be introduced to other members who are interested in the same Topics.
If you ever want to update or change which Topics you follow, just navigate to Topics from the left navigation and click or tap on "Following" or "Follow" to Follow or unfollow that Topic.
Here's how it looks on the web: