Rashi

להוסיף לכם תבואתה [AND IN THE FIFTH YEAR SHALL YE EAT THE FRUIT THEREOF] THAT IT MAY YIELD UNTO YOU ITS INCREASE — This command (of ערלה and נטע רבעי, fruit of the fourth year) which you observe will result in its giving you its produce in larger quantities. Because as a reward for it (for the fulfilment of My command) I shall bless for you the fruit of the plantation. R. Akiba used to say: “The Torah says this because it has man’s evil inclination in mind: that one should not say, "Behold, for four years I must take trouble with it for nothing!" Scripture therefore states that the result of your obedience will be that it will give you its produce in larger quantities (Sifra, Kedoshim, Section 3 9).
'אני ה‎‎‎ means, it is I, the Lord, Who gives you a promise about this (that your obedience to Me will bring a blessing on your harvest) and Who am faithful to keep My promise.