Each ring signifies one year of growth. Thicker rings generally mean there was more water that year than thinner rings.
Dendrochronology is the science of dating events and variations in the environment by studying and comparing the growth of tree rings. Extreme weather produces thicker or thinner and sometimes multiple rings. Rings from trees in the same area can be matched with others in a process called cross-dating, which has given us a history of climate and environmental conditions extending back over 13,000 years.