Helmholtz double layer

I don't quite understand what is meant by the Helmholtz double layer.
Textbooks states something along the lines of:



> 
> the colloidal particles attain charge by selective adsorption, due to
>  this it attracts a layer of opposite charge,this layer is called the
>  Helmholtz double layer and the potential between them is called Zeta Potential
> 
> 
> 


What stops the oppositely charged layers from combining (neutralizing) with each other?


