In the UK, your predicted grades are used in your application to universities. Most courses have a minimum grade requirement for your application to be considered—this is often an overall score (e.g. 36) and HL scores (e.g. 6 6 5). Some courses have specific subjects as well (e.g. you may need a 6 in Maths HL).
Your predicted grades are likely based on exams you have done and work you have completed, as well as your probable trajectory. They should be “aspirational but achievable”. This creates a fair basis for applications for university.
Most UK universities give conditional offers, which means you need to achieve a certain grade to get the university place. For instance, my offer for Oxford was 38 points with 6 6 6 at Higher Level.
You have a Firm and Insurance choice, usually chosen on the basis of a higher and lower conditional offer. If you don’t get the grades for your Firm but are good enough for your Insurance, you can go there.
If you don’t get the grades for either, there is Clearing, which means you can call up universities to get places on the courses where they still have space. There is also sometimes room for negotiation with your Firm and Insurance choices, so it is worth calling them up to see if they will take you anyway.
Universities differ widely on their approach to conditional offers and their flexibility. Cambridge gives out very high offers: usually 42 or above, sometimes as high as 45 or 7 7 7 at Higher.