A parser is the part of a compiler or interpreter that scans the program text (or any intermediate code or symbols) and breaks it down into tokens and symbols for translation, usually building a data structure.
It makes "parts" (lat. pars), that's why it is called parser (I think) :-)
That message seems to imply that the program has detected some inconsistencies in the parser, that tries to make an operation on an "unset target type".
It asks to report this as a bug.