Vehineh HASHEM nitzav alav - And behold HASHEM stood above it
According to the plain meaning of the text the word alav refers to the ladder.
The word alav refers to Yaakov, i.e. G-d appeared to Yaakov as if He were standing above him. What the Kabbalists are telling us is that Yaakov viewed himself in this dream as the carrier of the Shechinah (the Divine Presence), and its entourage. The Torah would thereby have compared Yaakov to one of the keruvim of whom it has been said "He mounted a keruv and flew" (2Shmuel 22:11) The Patriarchs are the true merkavah (chariots) supporting the Shechinah, the benevolent Presence of G-d. They performed a task on earth, which the angels perform in the celestial spheres. Kabbalists (true scholars, i.e. the Sages of the Talmud) have expressed this thus when speaking about the keruvim, "The face of a human being is considered the 'major' face whereas the face of the keruv is considered as the 'minor' face." (see Sukkah 5b where the keruvim on the lid of the Holy Ark are discussed).
According to the Kabbalists it was this idea David referred to in Tehillim 104:25 where he referred to chayot ketanot im-gedolot - "small and large chayot." As was previously stated (Bereishit Rabbah 68:12), Yaakov's face is engraved on the Throne of G-d and the angels descended to compare Yaakov's sleeping countenance with that engraved on G-d's Throne. This was the reason the Torah describes him as "dweller in tents" (25:27), i.e. he was "at home" both in a celestial residence as well as in a terrestrial one. This idea is expressed even more forcefully in connection with the revelation at Mount Sinai where the "nobles" of Yisrael (Shemot 24:10) are described as having a vision "of G-d, and Yisrael who is at His feet." This is also what Yaakov had in mind when he said to Rachel who had demanded that he make children for her (30:2) hatachat elokim anochi. This statement was not a rhetorical question but he explained to Rachel that he was not G-d-like, but his image was inscribed below, i.e. beneath the throne of G-d Who has the power to bestow children.