Here is the explaination of above command (Go through any awk tutorial to get a basic idea of how awk works.)
awk processes input file line by line and in each line, $0 represents the whole line, $1 represents 1st field, $2 represents 2nd field and so on. Default delimiter is space/tab.

Code:

echo "abc def ghi" | awk '{print $0}'

will prints whole line

Code:

abc def ghi

Code:

echo "abc def ghi" | awk '{print $1}'

will prints 1st field

Code:

abc

Code:

echo "abc def ghi" | awk '{print $2}'

will prints 2nd field

Code:

 
def

When more than one file is given to awk,

Code:

 
NR==FNR

will be true only for 1st file. FNR is record no in current file, NR is record no processed by awk (so commulative count).

Code:

 
NR==FNR{a[$2]=$0;next;}

This block will execute only for 1st file (As NR==FNR will be true only for 1st file). Here record value is being assigned to array (indexed with 2nd field i.e. [NodeID=1/2/3]). next command will get next line in the file for processing.

Code:

{print $0,a[$2]}

This block will execute for 2nd file ONLY.
Here $0 is the whole current record value in 2nd file and $2 is 2nd field in current line (i.e. NodeID in 2nd file). a[$2] will be printed if array was set for this index (2nd field in 2nd file) while processing 1st file.