“Goodly trees” refers to the beautiful or noble trees, branches, or fruit used during the Feast of Tabernacles. The original expression may have been general, though later Jewish tradition identified it with the citron.
The fruit or boughs of goodly trees were one of the four kinds of plants used in the Feast of Tabernacles, according to Leviticus 23:40.
The Septuagint and Vulgate understood the expression generically. The Talmud identified it as the citron, and this interpretation was ancient. The citron was probably brought from Babylon by Jews returning from captivity.