This is the first time I've seen such usage. Unfortunately, I can't find relevant information.
awk '{print $$1}'
What's $$1 for? Is it an array?
$ echo -e "2 2 3 4\n2 22 3 4 5 6\na 2 3" | awk '{print $$1}'
2
22
a 2 3
$ echo -e "2 2 3 4\n2 22 3 4 5 6\na 2 3" | awk '{print $$2}'
2
2
$ echo -e "2 2 3 4\n2 22 3 4 5 6\na 2 3" | awk '{print $$3}'
3
3
3
The above are just examples. Note that this is not a single $, it's a $$
Indirect field addressing
Look at this as like this $($1).
So for second line 2 22 3 4 5 6 this command awk '{print $$1}' will look at $1 (first field). It does contain value 2, then the result will be $2 (second field) so result of $$1 will be 22. $$1 will take the number from field #1 ($1) and use this as a field number for the outer $.
Example:
$ echo -e "1 b c d\n2 b c d\n3 b c d\n4 b c d"
1 b c d
2 b c d
3 b c d
4 b c d
Then:
$ echo -e "1 b c d\n2 b c d\n3 b c d\n4 b c d" | awk '{print $$1}'
1 # since filed #1=1 result print first field
b # since filed #1=2 result print secondfield
c # since filed #1=3 result print third field
d # since filed #1=4 result print fourth field
Edit
As @kvantour points out, you can add as may $ as you like
awk '{print $$$1}' = awk '{print $$($1)}' = awk '{print $($($1))}'
And as @Ed points out, you can make it more readable by expand it like this:
awk '{print $$1}' = awk '{fieldNr=$1; print $fieldNr}'