What is the meaning of: "Like a grazing animal beside the crop, it is about to extend its mouth towards it, and the crop can hardly escape it"?

A hadith (definition) with this exact wording has not been reported in the authenticated hadith collections. Rather, it is reported in the Two Sahihs and other collections, on the authority of Nu'man ibn Bashir, that the Prophet, peace and blessings be upon him, said: "The lawful is clear, and the unlawful is clear, and between them are doubtful matters which many people do not know. So whoever avoids the doubtful matters has cleared himself for his religion and his honor. And whoever falls into the doubtful matters is like a shepherd grazing around a sanctuary; he is about to enter it. Indeed, every king has a sanctuary, and indeed, Allah's sanctuary on His earth are His prohibitions. Indeed, there is a piece of flesh in the body: if it is sound, the whole body is sound, and if it is corrupt, the whole body is corrupt. Indeed, it is the heart."

It is also narrated from Aisha that the Prophet, peace and blessings be upon him, said: "The essence of religion is piety (wary)."

The wording mentioned in the question appears in the book "Futuh al-Ghaib" by Sheikh Abdul-Qadir al-Jilani, which is not considered an authenticated source for transmitting hadith.

The meaning of "like a grazer beside a crop, he is about to stretch his mouth towards it; the crop is rarely safe from him" is that an animal grazing near a crop will not avoid eating from it. Similarly, whoever approaches Allah's prohibitions rarely avoids falling into them.