Assuming I am understanding your question correctly, its the pin/bundle of wires that plug into the motherboard. They come from the PSU, and plug into the motherboard. The common one now is a 24pin connector. The previous common plug was a 20pin connector. It provides your motherboard with power.
explain what exactly is 24 pi eatxpwr
It's the main motherboard connector. It is the biggest of the cables that you need to connect to the mobo.