I have below sas code

data original_data;
    input x y z z1;
    datalines;
1 4 76 1
2 3 49 0
2 3 85 0
4 5 88 0
2 2 90 1
;
run;

/*view dataset*/
proc print data=original_data;


data original_data_1 ;
set original_data;
length New_Col $30.;
New_Col = 'AAA';
if (z > 80) then New_Col = 'BBB';

proc print data=original_data_1;

data data_2;
set original_data_1;
if first.z1 then zzz = "Y";
else delete;
by New_Col;
run;

proc print data=data_2;

I could not understand the part

if first.z1 then zzz = "Y";
else delete;
by New_Col;

What is basically happening here?

Any interpretation would be helpful