@+数字表示绝对控制符号,就是说某个变量,在某列或某些列的数据,还有其他的,你自己看
@@表示不变行的数据输入

I think you are talking the @/@@ at the end of input statement.
In short,
    input x y z @;           *** sas will hold this line
    input x y z @@;      *** sas will go to next line to get data if there is no data elements left in current line.

Here is a simple example. But you need read SAS manual to understand them better. I think 3/4 of sas programmer do not fully understand them.

data t1;
  input x y z @;
  output;
  input x y z @;
  output;
  cards;
1 2 3 1 2 3
4 5 6 4 5 6
;

proc print; run;

data t1;
  input x y z;
  output;
  input x y z;
  output;
  cards;
1 2 3 1 2 3
4 5 6 4 5 6
;

proc print; run;

data t1;
  input x  @@;
  output;
  cards;
1 2 3 1 2 3
4 5 6 4 5 6
;
proc print; run;

data t1;
  input x ;
  output;
  cards;
1 2 3 1 2 3
4 5 6 4 5 6
;
proc print; run;