yyrkroon is right.
There are three levels of message decoding, 0-2. Usually we want level zero, which is the easiest to read. Level 1 outputs raw data, and Level 2 is in HEX. To change the output message level, use:
SET MSGI x MON (0)-2
SET MSGO x MON (0)-2
Level 1 monitoring can give you way more raw information than you will ever need, but level 2 is quite useful at times. You will need to find a good hex conversion program or website too in order to convert the hex to information that you can understand. Unless you can natively read hexidecimal that is.
Do not forget to shut off monitoring after you are done using it or your history files will fill up fast.