lockfile--what is it?

[Steve Winston <xee42@netscape.net>]

What is lockfile /var/run/update-menus.pid?
In slink, when I run install after apt-get, etc., I
get the following message:
"update-menus -
cannot remove lockfile /var/run/update-menus.pid"
   I can't find any reference to this file. This has
just started to appear.
What is a lockfile anyway? How is this affecting my 
system's performance? Should I worry? Should I remove
it? How?
thanks, steve w.

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