pillar

This is a memorial pillar, that is, simply a large stone or boulder set up on its end.

poured oil upon the top of it

This action symbolizes that Jacob is dedicating the pillar to God. The full meaning of this statement can be made explicit. Alternate translation: "poured oil on the top of it in order to dedicate the pillar to God" (See: rc://en/ta/man/translate/translate-symaction and rc://en/ta/man/translate/figs-explicit)