Parallel tracking is a new way of handling tracking requests when implementing custom click tracking. It takes advantage of the new sendBeacon browser function to perform tracking in the background and thereby accelerate the user to their final destination. This helps reduce lost visits when a user clicks on your ad but never sees your landing page because they navigate away before the redirect from your tracking URL completes.
As of May 2018, parallel tracking is optional and works only for Search and Shopping campaigns from the Chrome browser on Android. Later in the year parallel tracking will be the only method available. Over the next several months we will enable support for additional campaign types and browsers. If someone views your ad on a browser that doesn't support parallel tracking yet, Google Ads will revert to sequential tracking.
With parallel tracking, the navigation experience is as follows: