TWOT stands for Theological Wordbook of the Old Testament edited by R. Laird Harris, Gleason L. Archer, Jr., and Bruce K. Waltke. TWOT is a 2-volume set, dealing with the Hebrew/Aramaic words in the Old Testament that have a theological significance. It gives a short definition to every Old Testament word, but goes theologically in-depth on the words that would be necessary. Though our Old Testament Lexicon references the TWOT numbers, we do not currently make the TWOT available through the Blue Letter Bible.
When using the lexicon on the Blue Letter Bible for the Old Testament, one of the references cited is the TWOT. Here is the TWOT reference for the word "bârâ'" :
TWOT - 278
The number refers to the entry number of the word. The word "bârâ'" is the 278th word in TWOT.
According to the introduction, "The contributors were asked to study their words from the viewpoint of biblical usage, etymological background, comparison with cognate languages, translations in the ancient versions, synonyms, antonyms, and theological significance" (Vol 1, iv). While the above was the focus of TWOT, not every major entry was able to include all of these.