Rashi on Deuteronomy 23:25/רש"י על דברים כ״ג:כ״ה

1כי תבא בכרם רעך WHEN THOU COMEST INTO THE VINEYARD OF THY FELLOW MAN, [THEN THOU MAYEST EAT GRAPES AT THY FILL] — Scripture is speaking of a laborer (who is engaged in gathering in the grapes, but not of one who is doing other work in the vineyard, nor of one who enters the vineyard with no intention to do work; cf. Rashi in the next passage) (Bava Metzia 87b).

2כנפשך means as much as you like.

3שבעך AT THY FILL — but not excessive eating (Bava Metzia 87b).

4ואל כליך לא תתן BUT THOU SHALT NOT PUT ANY IN THY VESSEL — From here we may derive that Scripture is referring only to the period of the vintage, to the time when thou puttest grapes into the owner’s vessel, — then thou mayest eat but not put any into thy vessel; but if he comes to hoe or to weed (i.e. to do other work than harvesting) he must not eat of the grapes (Bava Metzia 89b).