A numerical fraction has exactly one value. There are equivalent fractions, but they all have the same value (i.e. they represent the same point on a number line.)
A rational expression may take on infinite values. For certain values for the variables, the rational expression may not be defined. Like numerical fractions, rational expressions can have equivalent expressions; they can also have expressions that agree everywhere except at a finite number of points.
The graph of a (defined) numerical fraction is a point on a number line, while the graph of a rational expression can be a curve in a coordinate system.
Thanks so much for your answer. Could you give me an example of a rational expression and numeric fraction, where they are the same and where they are different.