Labels beginning with L (upper case only) are called local labels. Refer to Section 6.3 Symbol Names. Normally you do not see such labels when debugging, because they are intended for the use of programs (like compilers) that compose assembler programs, not for your notice. Normally both as and ld discard such labels, so you do not normally debug with them.
This option tells as to retain those L… symbols in the object file. Usually if you do this you also tell the linker ld to preserve symbols whose names begin with L.
By default, a local label is any label beginning with L, but each target is allowed to redefine the local label prefix. On the HPPA local labels begin with L$.