You can get the URLs for link tracking in Settings > Applications & integrations.
Scroll down to reveal the "Integrations" list. Click on "Show guide" under "Email tracking links" for the instruction manual to link tracking.
Personal emails sent from Gmail or Outlook with the sidebar enabled will have link tracking automatically.
The link tracking here is for use in MailChimp, MailShake, or other programs to send newsletters, transactional emails, or outbound emailing campaigns.
It helps to identify users with the website tracking as well.