Every year that a tree grows its trunk gets fatter. This is because trees don't just grow up they grow out in the form of rings too. The newest growth of a tree is between the bark and the wood that grew the year before.
In parts of the world with four seasons, trees usually grow best in the spring. Some parts of the world only have two seasons, wet and dry. In those areas, trees grow best in the wet season. Wood that grows in the spring makes a light-coloured ring. In the summer (or dry season) trees don't grow as much. Wood that grows in the summer makes a dark-coloured ring.
1 light-coloured ring + 1 dark-coloured ring =
This is called a "growth ring."
The growth rings in a tree don't all look the same. This is because climate and other things going on in the environment affect a trees' growth. The temperature, amount of rain, what the soil is like, wind, sunlight, amount of snow on the ground and insects all affect how a tree grows. These things are different from year to year.