it mean if the field ($4 or $5) contain the pattern in /pattern/
ex:-
($4 ~ /PP/ && $5 ~ /SIZE/ # means if $4 contains (or include) the word PP and $5
contains (or include) the word "SIZE"
BR
expr ~ expr ERE match numeric none
expr !~ expr ERE non-match numeric none
....
A regular expression can be matched against a specific field
or string by using one of the two regular expression match-
ing operators, ~ and !~. These operators interpret their
right-hand operand as a regular expression and their left-
hand operand as a string. If the regular expression matches
the string, the ~ expression evaluates to the value 1, and
the !~ expression evaluates to the value 0. If the regular
expression does not match the string, the ~ expression
evaluates to the value 0, and the !~ expression evaluates to
the value 1. If the right-hand operand is any expression
other than the lexical token ERE, the string value of the
expression is interpreted as an extended regular expression,
including the escape conventions described above. Notice
that these same escape conventions also are applied in the
determining the value of a string literal (the lexical token
STRING), and is applied a second time when a string literal
is used in this context.