What does the 'Insert Disable Tracking link' feature in the message editor do?

When you add the "Disable Tracking" link in the body of an email message, you allow recipients to turn off all tracking on that message.

In other words, if they click on that link, the fact that they opened the message and that they may click on one or more links, will not be tracked.

Note that this disables tracking only for the recipient that clicks on the link, and only for the message in which they click on that link.

If you wish to disable tracking on all messages, for all recipients, you can do so by turning off link tracking at the list level, in the advanced settings of the list.