What is a "regular (endocentric) thematicization"?

From: r_brunner
Message: 71273
Date: 2013-08-18

The Wiktionary entry at http://en.wiktionary.org/wiki/war#Tocharian_B for the Tocharian B word for "water" gives a PIE form *udrom as the probable origin for this and states that *udrom is a "regular (endocentric) thematicization".

I could not find out what this means. Does this say that already in PIE there might have been a special form *udrom of the "water" word and say something how that form might have developed?

I traced the info back to the entry for "war" in "A Dictionary of Tocharian B" by Douglas Q. Adams, so it's probably reasonably up-to-date, but that entry does not include more explanation concerning this.

And what is *udrom in the root-suffix-ending scheme? *ud-r-om? With what roles for suffix "r" and ending "om"?