Think of a serial like a TV show with an overarching arc, such as the X-Files or Lost. Like TV series that are divided into episodes and seasons, a serial story will be divided into novelette- or novella-sized sections released at regular intervals, like once a week or once a month.
Some series are only one season long (stories that span one novel), and some are multiple seasons long (stories that span many novels). Series (and serials) that feature multiple seasons (multiple novels) tend to have both seasonal arcs and overall series arcs: stories whose threads are tied in one season (one novel), and stories whose threads aren’t tied until the series is complete.
Of course, each episode within a season has its own arc too, but those arcs are often much smaller and simpler and cannot generally stand alone. They’ll also often end on a cliffhanger or a note of suspense regarding the season and/or series arc.