"It gives its fruit" - meaning its produce - "every season by the permission of its Lord," meaning: by His command. Al-Hasan's interpretation is: He says that the believer continues to produce good speech and righteous deeds, just as this tree yields its fruit at every season.
Yahya said: "The 'hin' (season/time) in the interpretation of some of them is: a year, and it is consumed in both winter and summer.
Muhammad said: "The 'hin' in the language is: a noun for a time among the times of duration; it is used for both long and short periods."