LET ME EXPLAIN! I HAVE BEEN WAITING FOR SUCH A QUESTION!
Story arc is like a division o plot within a story. To put is simpler, there are mini plots inside the general plot of the story.
For example let's take a look at the most popular series, Harry Potter.
We know that a general plot is related to defeating Lord Voldemort and that's what seven books emphasize.
However, each book speaks about a different problem that must be dealt with and each one pushes the srtory in a concrete direction. .
For instance, The first book deals with Philosopher's stone. So the plot revolves around finding and protecting the stone. So this is the first arc.
The Second arc could be "The chamber of secrets" which focuses on the Basilisk and Tom Riddle.
Each books deals with a different problem and we could call it a story arc.
So to summirize, story arcs are plots within the main story that deals with a particular porblem. It helps with character development if we talk about adventure stories.
I hope I was clear enough