when you write up the net ionic equation, 

we separate ionic species in aqueous solution into ions, right? 

But only strong electrolytes can be separated like NaCl or AgNO3, 

when you write the ions as separated, 

some ions appear in both sides as completely same, 

those ions are not reacted, so they are cancelled in net ionic equations, 

they are called spectator ions, since they've not participated in a reaction. 

I hope you understand .

:)