chrome/common/extensions/docs/templates/articles/manifest/key.html

<h1 id="key">Manifest - Key</h1>

<p>
This value can be used to control
the unique ID of an extension, app, or theme when
it is loaded during development.
</p>

<p class="note">
<b>Note:</b> You don't usually need to
use this value. Instead, write your
code so that the key value doesn't matter
by using <a href="http://developer.chrome.com/extensions/overview#relative-urls">relative paths</a>
and <a href="http://developer.chrome.com/extensions/extension#method-getURL">extension.getURL</a>.
</p>

<p>
To get a suitable key value, first
install your extension from a <code>.crx</code> file
(you may need to
<a href="https://chrome.google.com/webstore/developer/dashboard">upload your extension</a>
or <a href="http://developer.chrome.com/extensions/packaging">package it manually</a>).
Then, in your
<a href="http://www.chromium.org/user-experience/user-data-directory">user
data directory</a>, look in the file
<code>Default/Extensions/<em>&lt;extensionId&gt;</em>/<em>&lt;versionString&gt;</em>/manifest.json</code>.
You will see the key value filled in there.
</p>