Bison is a tool used to write parsers, such as the parser for GNU cc.
It is similar to Yacc, which is included in the base FreeBSD system.

The main difference between Bison and Yacc that I know of is that
Bison supports the @N construction, which gives you access to
the starting and ending line number and character number associated
with any of the symbols in the current rule.

Also, Bison supports the command `%expect N' which says not to mention
the conflicts if there are N shift/reduce conflicts and no reduce/reduce
conflicts.

The differences in the algorithms stem mainly from the horrible
kludges that Johnson had to perpetrate to make Yacc fit in a PDP-11.

Also, Bison uses a faster but less space-efficient encoding for the
parse tables (see Corbett's PhD thesis from Berkeley, "Static
Semantics in Compiler Error Recovery", June 1985, Report No. UCB/CSD
85/251), and more modern technique for generating the lookahead sets.
(See "Efficient Construction of LALR(1) Lookahead Sets" by F. DeRemer
and A. Pennello, in ACM TOPLS Vol 4 No 4, October 1982.  Their
technique is the standard one now.)