Tu B’Shvat, the 15th day of the Jewish month of Shvat, is a holiday also known as the New Year for Trees. The word "Tu" is not really a word; it is the number 15 in Hebrew.
Tu B’Shvat is the new year for the purpose of 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 that, you can eat the fruit. Each tree is considered to have aged one year as of Tu B'Shvat, so if you planted a tree on Shvat 14, it begins it second year the next day, but if you plant a tree two days later, on Shvat 16, it does not reach its second year until the next Tu B’Shvat.
There are few customs or observances related to this holiday. One custom is to eat a new fruit on this day. Some people plant trees on this day. A lot of Jewish children go around collecting money for trees for Israel at this time of year.
Source: Bard, Mitchell G. Jewish Holidays: Tu B'Shvat 1998