What is the significance of the Matzevah that Yaakov set up?
Rashi (28:22): Having set up the stone as a Matzevah on his way to Charan, and having promised to worship Hashem on it, 1 he now erected a Matzevah and poured a libation on it, 2 in fulfillment of his Neder.
Ramban (citing the Ibn Ezra) and Seforno: The Torah recalls that on his way to Charan he set it up Matzevah, and he now offered on it . 3
Malbim: He set up a Matzevah, which is one rock, for this is related to the prophecy, that Even Yisrael will be one. The libation on it represented clinging and Divine influence.
As Targum Onkelos explained there.
Ha'amek Davar: The rock he erected 22 years ago had been displaced. He took another rock from the area; earlier, he had said "Yihyeh Beis Elokim" to be Mekadesh all the rocks there. Oznayim la'Torah - This is the same rock he set up earlier. (Now, he arranged it (EC)).
Seforno: He prepared it for the future, when the Beis-ha'Mikdash would be built on it, thereby fulfilling the promise he made on his way to Charan ("And this stone which I placed as a Matzevah will be the House of G-d" (28:22)) - like David ha'Melech did when the angel appeared to him in the granary of Aravnah ha'Yevusi (Shmuel II 24).
What libation did Yaakov pour on the Matzevah before he poured oil on it, and why?
Targum Yonasan: He poured wine and water on it, corresponding to the wine and water libations that his children would later pour on the Mizbe'ach on Sukos.
Ibn Ezra He poured wine or water on it to wash it, before pouring oil on it.
Ha'amek Davar: Nesachim represent supporting Chachamim (Yoma 4a). Yaakov showed his Midah of including Torah and Chesed.
