I presume you’re referring to the “Scores on the Doors” online database - formally known as the National Food Hygiene Ratings Scheme.
You’ll see ratings from 0 - 5, as a summary of inspections findings by the local council’s Environmental Health Officers. I don’t know the specific details as to what an EHO actually checks when s/he visits premises but I assume it will mainly be checking the actual premises for any structural matters, together with checks on how food is stored and prepared. I suspect we’ve all seen TV programmes titled along the lines of “Britain’s Horror Restaurants” ( almost invariably including a seedy looking fried chicken takeaway place). The website has further explanation, and a slightly wider description of the issues at particular places - rating them from, for example, “very bad” to “very good”, in terms of their compliance with food safety & hygiene, structural matter and “confidence in management”
As far as I know, restaurants are not obliged to display their ratings, so if you just walk up to a place you have no way of knowing if that’s because the owner simply hasnt bothered or if s/he’s not displaying because it’s a low score. I think that’s part of the scheme that needs amending - although I do understand that there can be delays between a place undertaking necessary improvements and the inspector revisiting. That happened to a well known place near me where there had been some construction work that had adversely affected the rating - it was fixed within days but the low score was not amended for weeks (it was a matter that hit the local press).
Now, personally, I prefer not to look at the scores before going to a new place - I’m visiting because something has tickled the foody in me and I really want to try it. If I visit somewhere and have concerns, then I will check the scores and it would then help me decide whether to go back. For example, I used to visit a couple of Sichuan places in Manchester fairly regularly - both well respected, with one appearing in the Good Food Guide (and had been reviewed by Jay Rayner). I’d enjoyed the food and had no concerns about them. However, on a subsequent visit , the toilets (which I’d used on previous visits) were in very poor condition - I’d suggest it had been days since they had received attention. I always reckon poor attention to toilet areas is likely to indicate poor hygiene practice in areas the customers don’t see. Checked the scores for both place to find them being a 1 - “major improvement necessary”. I’ve not been back.
It is hard to discuss this matter without bringing politics into it. The number of environmental health officers have been cut over the recent years of austerity. It means that they cannot inspect premises as often as might be wished. That can mean poor practice going unchallenged for much too long. I know local councils face very difficult choices - do they keep teachers, child protection social workers, or EHOs - and I’m glad I don’t have to make the calls. But I do think food inspection is important - there are few things as important as the safety of the food we put in our mouths.