vim help =

                            *=*
={motion}       Filter {motion} lines through the external program
            given with the 'equalprg' option.  When the 'equalprg'
            option is empty (this is the default), use the
            internal formatting function |C-indenting| and
            |'lisp'|.  But when 'indentexpr' is not empty, it will
            be used instead |indent-expression|.  When Vim was
            compiled without internal formatting then the "indent"
            program is used as a last resort.

                            *==*
==          Filter [count] lines like with ={motion}.

                            *v_=*
{Visual}=       Filter the highlighted lines like with ={motion}.
            {not in Vi}