We all say a electron has a negative charge and proton has a positive charge.But I need to know what does a charge of a electron mean?Please can anyone explain this matter?

Consider a point particle in an electric field [math]\boldmath{E}[/math]. Then the charge is the ratio of the magnitude of the force experience by the particle and the magnitude of the electric field applied.

 

[math]q = \frac{|\boldmath{F}|}{|\boldmath{E}|}[/math].

A charge means it interacts electromagnetically. As a result of electric fields, as ajb mentions, or magnetic fields if it's moving with any velocity component perpendicular to the magnetic field.