r1 is the rafsi prefix used in stage-3 fu'ivla v1=f1, rafsi based on word r2.
“rafsi prefix” is a term specific to Lojban and refers to the initial part of a stage-3 fu'ivla which is a rafsi. x2 is the word in which the rafsi is used, x3 is the word from which the rafsi has been derived. See also: cibyfu'ivla, fu'ivla, rafsi.