I'd not heard of it either but I googled - and the first few results were all from or about Bristol so maybe it's Their Thing 
From the university themselves:
Putting an application on hold means that we have assessed the application and decided that the applicant meets the entry requirements for our offer, but we are unable to make a final decision.
We may need to hold off on making a final decision or issuing an invitation so that:
we can assess more applications;
we know how many offers we can make to fill the available places;
we know how many interview invitations to issue to ensure fairness;
we can interview more applicants to ensure fairness across the entire interview period.
We often put applications on hold until after the UCAS application deadline so that we can consider all applications fairly. Making too many offers early in the cycle might mean that we have too many students to fit into our teaching space or accommodation.
We may put applications on hold until we know how many of our offers are being accepted. This gives us the best chance to get the right number of students on each course, ultimately to provide our students with the best university experience.
Effect on other applications
Putting your application on hold won't affect your other applications. You can only reply to your offers when you have received all your decisions. If another university has made you an offer, they won’t withdraw it or change it just because you haven’t had a decision on one of your other applications.