TDP (Total Dissipated Power) indicates the maximun amount of heat power that the CPU cooling is able to manage without damaging the internal structure of the transistors.
So TDP actually has and does not have an effect on overclocking. A higher TDP in a CPU would mean that you need a better cooler that is able to mantain a certain temperature. If you overclock it, so the power consumption of the CPU increases, you will need a cooler better to deal with it. So it does not influence the OC directly, but it is something to have in account.
Overclocking consists in increasing the clock frequency of the CPU, GPU or any clock-based electronics. You can possibly increase the frequency a little without having to modify the voltage, but as the...