Untracked transactions are essentially a "transaction claim" that a publisher has made as a result of a sale not tracking. This could be for a few different reasons, such as the implementation of a tracking link not being done properly by the publisher, or an issue with your (the advertiser) tracking code. Sometimes adblockers can be a factor too, and block the tracking cookie. Whilst the majority of transactions do track through, any that don't can be submitted by the publisher so that they can still get their commission.
You might see this a little more often with cashback publishers, as the customer is querying their 'missing' cashback, so the cashback publisher will upload it in the platform.
Pending untracked transactions will need to be approved, declined, or edited within 30 days or they will be automatically approved.
If you have new pending untracked transactions, you should get an email notification, but there is also a widget box on your account homepage showing the number of pending. If you click through this, or go to the reports tab > Check Transactions > Untracked Transactions, you can easily action any.