Jeremy Herve on "[Plugin: Jetpack by WordPress.com] Jetpack crashing all of my sites"

I don't know how to disable the WP_DEBUG constant (#) or even what that means.

The WP_DEBUG constant is a setting that can be used to enable the "debug" mode throughout WordPress. It's meant to be used while creating a theme, or a plugin, and is off by default.
Unfortunately, it seems to be activated on all your sites.

To deactivate it, you will need to edit your sites' wp-config.php file. This file can be found at the root of your WordPress installation.

To edit it, log in to your site via FTP (more information here), open the file, and look for that line:

define( 'WP_DEBUG', true );

Change it the following:

define( 'WP_DEBUG', false );

And then save your changes. The notices should immediately disappear.