If you have not visited the site for a while, you may want to catch up on what has been happening: new topics? new replies? new members? etc.
You can view all the activity in reverse chronological order using the Activity menu tab. This is a good way of catching up.
If you want to keep something from this list in mind, you can mark it as a favourite and later display just your marked activities. Note that marking activities as favourites is distinct from marking topics as favourites.
You can delete your own activity entries if you would prefer them not to be included. If you have edited a topic or comment, this will be listed as well as the original activity and you might want to delete one of them as superfluous.