Question

What can you learn by looking the cross section of a woody stem?

Answer 1

Secondary xylem is commercially important as wood. The seasonal variation in growth from the vascular cambium is what creates yearly tree rings in temperate climates. Tree rings are the basis of dendrochronology, which dates wooden objects and associated artifacts. Dendroclimatology is the use of tree rings as a record of past climates. The aerial stem of an adult tree is called a trunk. The dead, usually darker inner wood of a large diameter trunk is termed the heartwood and is the result of tylosis. The outer, living wood is termed the sapwood.
for more reference please in wikipedia.

Answer 2

Summarizing the above, you can get a very close idea of what the age of the tree is. The above explanation by dips also gives you an explanation as to WHY there is one ring corresponding to one year in the life of the tree.