A test for correctness that all lujvo must pass. Tosmabru is a hypothetical lujvo, as if from tos+mabru, that fails the test: it falls apart into "to smabru". To fix this, a y hyphen is inserted, leading to the morphologically correct lujvo tosymabru.
Full details are available at http://www.lojban.org/files/reference-grammar/chap4.html#s11.