This error will come when you try to logout or exit from a terminal and there are some tasks which are running in background.
surendra@linuxnix.com:~$ ping linuxnix.com
PING linuxnix.com (216.239.38.21) 56(84) bytes of data.
64 bytes from any-in-2615.1e100.net (216.239.38.21): icmp_req=1 ttl=50 time=61.3 ms
64 bytes from any-in-2615.1e100.net (216.239.38.21): icmp_req=2 ttl=50 time=63.2 ms
^Z
[1]+ Stopped ping linuxnix.com
surendra@linuxnix.com:~$ exit
There are stopped...