This is one of the default queue tables used in replication. I assume you will knacker your replication if you truncate it.

If you have room in another tablespace then there is room for more SYSTEM tablespace. Simply reduce the size of another tablespace and make room for SYSTEM to expand. If the room is on another mountpoint then add a datafile to the system tablespace on that drive and turn the autoextend off the first file.

It's not a good idea to start messing with the SYS schema.

Cheers