The apply_filters() function can be used to create a new filter hook by simply calling this function with the name of the new hook specified using the $tag parameter. The callback functions attached to filter hook $tag are invoked by calling this function.
$value = apply_filters( $tag, $value, $var_1, $var_2, $var_3, ... );
You can use apply_filters to filter a given $value - with relation to the value itself and optionally provided variables inside the range $var_1 to $var_n.
Please refer here for more official info on this.