Serious, grave, solemn, sedate, staid, sober, earnest mean not light or frivolous. Serious implies a concern for what really matters. Please don't laugh - i'm being serious.
He was wearing a very serious expression and i knew something was wrong. On the surface it's a very funny novel but it does have a more serious. See examples of serious used in a sentence.
Serious can be used to describe something that is significant, grave or of significant importance.