I think its a photography of uncover wooden block which you caught wne some light is falling on it like this.
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But i think you are asking about the rings appearing on the wooden surface. These rings are called as ANNULAR rings. These rings are formed due to seasonal variations. In one season the tree stores food materials and resins in abundance with a dark ring and following a starving season with a thin ring.
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These are helpful in estimating the age of the tree. This study is known as dendrochronology.