Thermal Design Power, or TDP, is a statistic that is expressed in watts. It’s an expression of the amount of power a processor is expected to dissipate to prevent overheating. TDP is a self-reported metric. Each manufacturer announces it based on their own findings.
TDP is primarily used as a guideline for manufacturers of thermal solutions (heatsinks/fans, etc) which tells them how much heat their solution should dissipate. The processor TDP specification is very important because your thermal solution (heat sink/fans, etc.) must be able to be able to dissipate the rated TDP value.