Series are a collection of fics that the author has declared as being connected. Mostly if they take place in the same AU/Universe or if they are all part of a larger story.
You can see if a work is a part of a series in the info panel. It lists the series title + what number part the fic is in the series. Click the series title to get to the series page where you can bookmark/subscribe to the series and view all the other fics in the series.
You can also see if it’s a series at the bottom of the search page:
Example series page:
Authors can also add notes in the info box about recommended reading order + whatever else