packages have been kept back?

[RAccess <pi@home.com>]

What does it mean when I get the following:
***
debian:~# apt-get -u upgrade
Reading Package Lists... Done
Building Dependency Tree... Done
The following packages have been kept back
  task-devel-common
0 packages upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
***

What does this mean? It has been kept back for how long? Could someone
explain this in terms of what is happening to the package dependencies and
such.

Thanks.

RAccess
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