What does power on pword mean and why do I have one?

I have a lap top that is connected through wireless connection which is in another room. Ever since a new hard drive was put in my lap top it has asked me for a power on pword. I put it in and it starts. It is not my pword to start windows. I had another issue which may not be related so I will ask another ques. for that but I'm just curious as to what this poweron pword is and why I have it?