The Hebrew words in question are עזר כנגדו (ezer kenegdo).
The Hebrew root עזר means “help” and the word kenegdo comes from the root word נגד (neged). Neged in the OT always means "opposite" or "across from" and negdo means across from him. In Exodus 19:2, Israel encamp neged hahar, opposite to Mount Sinai.
The form kenegedo doesn't appear anywhere else in the Old Testament but is often used by the Sages of the Talmud to mean “corresponding to.” In his "The Art of Biblical Narrative," Robert Alter offers the translation “a sustainer beside him” and the more literal translation: "an aid opposite him."
Here is some of Alter's discussion of this passage:
"It is not good for man to be alone. I shall
make him a sustainer beside him (2:18)." [Following this verse] there occurs a peculiar
interruption. We have been conditioned by the previous version of
cosmogony to expect an immediate act of creation to flow from the
divine utterance that is introduced by the formula, "And God said."
Here, however, we must wait two verses for the promised creation of a
helpmate while we follow the process of the first human's giving names
to all living creatures. These verses (Gen. 2:19-20) are marked, as a
formal seal of their integration in the story, by an envelope
structure, being immediately preceded by the thematically crucial
phrase 'ezer kenegdo (literally, "an aid opposite him"), and
concluding with the same phrase...
Eve has been promised. She is then withheld for two carefully framed
verses while God allows the human creature to perform his unique
function of bestower of names on things. There is an implicit irony in
this order of narrated events. Man is superior to all other living
creatures because only he can invent language, only he has the level
of consciousness that makes him capable of linguistic ordering. But
this very consciousness makes him aware of his solitude in contrast to
the rest of the zoological kingdom...
His whole exposition is extremely beautiful and enlightening and I highly recommend reading it. Available on google books here.