what does the -@ mean when zipping a file in unix?

If you run man zip you can read:

-@ file lists.   If  a file list is specified as -@, zip
       takes the list of input files from standard input instead of  from  the
       command line. 

You can refer to this question on Unix & Linux Stack Exchange to better understand the difference between standard input and command line argument.

The manual has this option described as (see man zip from command line or an online version)

 -@     Take the list of input files from standard input.  
        Only  one  filename per line.

So to explain your command. This will have an undetermined amount of results:

 find . -type f -mtime -29 -mtime +21

and the result of this is accepted by the 2nd part of the command but only 1 result at once but still processed to the same zip file.

zip -@ 061110_061201.zip

according to man zip

-@ file lists.   If  a file list is specified as -@ [Not on MacOS], zip
   takes the list of input files from standard input instead of  from  the
   command line.  For example,

          zip -@ foo