What is a convention?
In terms of cinematic conventions, these are things that occur very often in movies,  so that they have become conventional. Particularly in certain genres, there are elements the come to define their styles.

What is the Romance Genre?
Romance films are typically defined as a genre where the plot revolves around the love between two protagonists. This genre usually has a theme that discovers a problem within love e.g forbidden love. The tone of this genre can vary whether the end is happy or tragic, but always manage to evoke strong emotions.

Examples of Romance films -
The notebook
If I, stay.
The Vow.
Love, Rosie.
LOL.
One Day
In Time
Titanic