Roger, besides being Pirate King, was well known for the wealth he had accumulated. Prior to his death, he's questioned as to the whereabouts of his wealth⁽¹⁾ where he proceeds to gives a rather cryptic response, and it is this wealth that he accumulated that is the One Piece...
The Will of D
Source: Chapter 100
What is a Will?
Wills are passed on after death; a will is an instruction or task imposed on specific person(s), and in some cases, a will is an unfinished project or goal that one decides to execute irregardless of not being specifically instructed.