checkpoint segments

Hi,

I have a small database on my Windows XP, I rarely use it. Even so, the log
file show says this:

[2005-07-02 02:02:09] LOG:  received fast shutdown request

 [2005-07-02 02:02:09] LOG:  checkpoints are occurring too frequently (0
seconds apart)

 [2005-07-02 02:02:09] HINT:  Consider increasing the configuration
parameter "checkpoint_segments".

 [2005-07-02 02:02:09] LOG:  shutting down

 [2005-07-02 02:02:10] LOG:  database system is shut down

 [2005-07-02 02:02:13] LOG:  logger shutting down


My checkpoint_segments setting is default, 3, and there are 2 wal log files.
Why does it say, that "too frequenty"?
In fact what are checkpoints? These files? I couldn't understand it exactly
from the docs.

Thanks,
Otto