Understand message before updating a module If I go to admin/reports/updates I see modules that need to be updates.

I just need to know what is the difference between the message "Recommended version:..." and the other message with red background. I mean what are the risk of not updating them?
The message on red background are critical or security fixes means the module is insecure or has known vulnerabilities. It's important to you to update flagged (red highlighted) modules as soon possible so you can prevents hacks.

A recommended module means your modules are many version(s) behind the current release, the module is known to be secure, and simply not up to date with features the current module offers.