Max is a function, that takes the maximum of the two arguments.
In this case for every row, it evaluates to the maximum of 815 and the value of X1 variable.
If you don't know what the parts mean, break out the code so you can examine it line by line. Here's an example. Look at the values that the check_flag's take over your data set, specifically what the start and ending for the do loop would be, and if they're what you intended.
data sasuser.ggp2;
set sasuser.ggp1;
array _v{*} v:;
if x1>1 then check_flag1=1;
else check_flag1=0;
check_flag2=max(815, x1);
check_flag3=min(dim(_v), xx1);
check_flag4=dim(_v);
run;