${_+_} refers to a variable. It is defined in share / mk / sys.mk , which is read by the make process. Therefore, user Zack indicated the right direction in his comment; this variable in some cases expands to the + sign, depending on the flags set for make:
.if !empty(.MAKEFLAGS:Mn) && ${.MAKEFLAGS:Mn} == "-n" _+_ ?= .else _+_ ?= + .endif
The rationale for this can be found in the comments for this commit and this one where the symbol was entered:
Make make recurse into sub-directories and sub-makes when given two -n flags. If only one -n flag is given the old behaviour is retained (POLA).