$A$ transverse section of the trunk of a tree shows concentric rings which are known as growth rings. How are these rings formed? What is the significance of these rings?

$\Rightarrow$ Concentric rings,also known as annual rings,are formed due to the activity of the vascular cambium during secondary growth in dicotyledonous trees.
$\Rightarrow$ In the spring season,the cambium is very active and produces a large number of xylem elements with wider vessels,forming 'spring wood' or 'early wood'. In the winter season,the cambium is less active and produces fewer xylem elements with narrow vessels,forming 'autumn wood' or 'late wood'.
$\Rightarrow$ The combination of one band of spring wood and one band of autumn wood constitutes an annual ring. By counting the number of these annual rings,the age of the tree can be estimated. This field of study is known as dendrochronology.