Tehillim 118:1 Give thanks to HaShem for He is good, for His lovingkindness endures forever.
King David composed the present psalm in thanksgiving for the future acts of deliverance on behalf of the Jewish people. Thus he both begins and ends it with "Give thanks to HaShem" (cf. v. 118:29). This conveys the importance of the present psalm and the things mentioned in it.
The psalm pertains to the Says of Mashiach, David prepared it for Jews in exile to chant in the future.
"For His loving kindness endures forever." Unlike kindness of a human being, whose time span is limited, the loving kindness of the Almighty will never terminate.
What is the intention of "For His lovingkindness endures forever" It is that the Holy One acts with lovingkindness towards Yisrael, not for one year, nor for two years, but "forever."
The Torah is called "lovingkindness," and it will never be taken away from Yisrael. For G-d has given the Torah as "an inheritance of the assembly of Yaakov" (Devarim 33:4)
Therefore, "Let Yisrael say: 'For His lovingkindness endures forever!'