Well, those are elements of the YAML file format, which is used here to provide a configuration file for configtxgen. The “&” sign mean anchor and “*” reference to the anchor, this is basically used to avoid duplication, for example:
person: &person
name: "John Doe"
employee: &employee
<< : *person
salary : 5000
will reuse fields of person and has similar meaning as:
employee: &employee
name : "John Doe"
salary : 5000
another example is simply reusing value:
key1: &key some very common value
key2: *key
equivalent to:
key1: some very common value
key2: some very common value
Since abric/common/configtx/tool/configtxgen/main.go uses of the shelf YAML parser you won’t find any reference to these symbols in configtxgen related code. I would suggest to read a bit more about YAML file format.