AWK : In Built Variables and Functions

NEXT and GETLINE 

The next statement forces awk to immediately stop processing the current record and go on to the next record. This means that no further rules are executed for the current record, and the rest of the current rule’s action isn’t executed.

Contrast this with the effect of the getline function. That also causes awk to read the next record immediately, but it does not alter the flow of control in any way (i.e., the rest of the current action executes with a new input record).

NR and FNR

NR - stores the total number of input records read so far, regardless of how many files have been read. The value of NR starts at 1 and always increases until the program terminates. 

FNR - stores the number of records read from the current file being processed. The value of FNR starts at 1, increases until the end of the current file is reached, then is set again to 1 as soon as the first line of the next file is read, and so on.