x1 (number) is the lujvo score of lujvo x2 (text) determined under scoring algorithm x3.
"lujvo score" is a term specific to Lojban lujvo. Lujvo can be assigned a "score" by some algorithm in order to compare those lujvo. One lujvo scoring algorithm is described in The Lojban Reference Grammar, chapter 4, section 12: https://lojban.github.io/cll/4/12/ See also vamji.