An ionic equation is an equation that shows the chemical species that actually take part in a reaction.
Ex. Pb(NO3)2 (aq) + 2KI (aq) -> PbI2 (s) + 2KNO3 (aq)
So the ionic equation for this reaction is
Pb2+ (aq) + 2I- (aq) -> PbI2 (s)
How do we know that Pb2+ and I- takes part in the reaction? What about K+ and NO3- ?