If you look at the rings from several trees in a forest, you will find the same pattern of thick and thin rings in each. If you looked at the rings of a 200 year old tree, it would give you a pattern that you could match with other trees from the area. But how do you go back beyond that tree?
The next step is to look at old, wooden structures in the area. If you found a log that was cut 175 years ago, from a tree that was 150 years old when it was cut, then you can extend the pattern. The last 25 years of its ring pattern would match up with the first 25 years of your first tree's ring pattern. The other 125 years of ring pattern on the second tree now becomes part of a pattern that covers a total of 325 years.
In an area such as Mesa Verde, where you have very old trees to start with, and many archaeological sites that contain logs of various ages, dendrochronology lets scientists identify ring patterns that cover huge spans of time. When new archeological sites are found, ring patterns from logs at the site can help tell us how long ago the logs were cut.