It means that everything you see is fake and unreal. If you can keep your heart, when you see these appearances, you can not be confused by these appearances, you can realize that what you see is not the real one, and then you can achieve Tathagata.
"Everything is illusory. If you see the opposite phase, you will see the Tathagata. " This is from King Kong Prajna Paramita Sutra.
A brief introduction to King Kong Prajna Paramita Sutra
King Kong Prajna Paramita Sutra, translated by Kumarajiva (later Qin Dynasty) and printed in the late Tang Dynasty and the Five Dynasties, is a precious ancient book in China, which is now in the National Library.
King Kong Prajna Paramita Sutra is an important classic of Mahayana Buddhism. The full name of "Can Break King Kong Prajna Paramita Sutra" (vajra cchedikāpraj? Pā ramit ā Sanskrit Interpretation: Breaking King Kong to the other side of wisdom), referred to as the Diamond Sutra. The French version of King Kong Prajna Paramita Sutra translated by Kumarajiva at the end of Qin Dynasty was the earliest and most widely circulated version.