calculating the age of trees for tithing.
Lev. 19:23-25 states that fruit from trees may not be eaten during the first three years, the fourth year's fruit is for God, and after the fourth year people can eat the fruit. Each tree is considered to have aged one year on Tu B'Shevat.
Tu B'Shevat is not mentioned in the Torah, and is an interpretation of the text.
Some people eat a new fruit on this day, while others plant trees. A lot of Jewish children collect money for trees for Israel at this time of year.