A story arc is the plot of your novel. More specifically, a story arc charts the trajectory of the events of your novel — the highs and lows, the conflicts and resolutions. They are called arcs because the path from point A (the beginning) to point B (the ending) is never a straight line. (And what fun would that be if they were?)
Conflicts must arise during the course of your novel. Complications, too. What's more, when one set of conflicts or complications is resolved, a new set must arise to take their place.
When plotting your romance novel, try using poster board, a black marker, and a stack of sticky notes. Draw squares for each chapter of your book on the poster board. Write down an element of your story arc on one of the sticky notes — such as “first kiss” — and place the note inside a square. Move the notes around until you find the right fit.
Depending on its size and format, a romance novel can have multiple story arcs. One will be for the romance itself, the others would be for any subplot(s) that the novel may contain. The subplots must support the main romance plot — either by adding further conflict for your main characters or by adding more insight into their psyches.
Each story arc will have three main components:
Introduction of the plot/subplot
Development of the plot/subplot
Resolution of the plot/subplot
The number of manuscript pages needed for the introduction and resolution of your plot or subplot will likely be much fewer than the number needed for its development. Ideally, your subplot(s) will support your main plot, by not only strengthening it but by “filling in” when the action has slowed for your main plot.
Does every scene have to support the romance?
Yes, although it may only do so indirectly. For example, if you have a suspense subplot and scenes from the villain's POV wherein he (or she) is plotting to kill the heroine, the danger the heroine faces is an obstacle to the romance with the hero.
The story arc for the romance between your two main characters will extend across the entire length of the book and will include every event, big and small, that contributes to the development of the romance and its Happily Ever After.
Story arcs for your subplots can be much shorter. Although they may only extend across several chapters, they will have the same components as your main plot.