All people invited to a Topic are Participants.
When you're invited to a topic, you'll receive a notification about the invitation. Once you click the notification, you're automatically watching the topic. In other words, you're following the topic thread. As a topic watcher, you're constantly up-to-date with what's being discussed in and you may contribute to the topic conversation. If at some point you're no longer interested in the topic, you may unwatch it. In that case, you won't be receiving any notifications on topic updates because you're no longer watching/following the conversation. Basically you're still a topic participant - only you're not actively involved in the conversation. You may start watching the topic at any point later on. You will be able to pick up from where you left off and catch up with what's been going on in the topic since you unwatched it.
You can only Close a Topic when you are the person who created (opened) it in the first place.