At a minimum, a website has a ‘root domain name’ (‘bmon’ in ‘bmon.co.uk’) sitting on a top-level domain extension (the ‘co.uk’). But the domains can also be divided into subdomains, which are indicated by a third part before those two. Back at the dawn of the internet, websites were primarily a medium to be used within an organisation, and if there was an outward-facing element, available on the World Wide Web, this would be given a subdomain of ‘www’.
Of course, we all know what happened, and the World Wide Web went stratospheric, with most sites only being set up as outward-facing. This meant that the ‘www’ subdomain was the only one on most domains, making that element rather redundant. It’s a bit like having a ‘Documents’ directory on your PC, and putting another ‘Documents’ directory inside this which holds all the files.
Over the years, although websites lived in the ‘www’ subdomain, people started to drop saying the ‘www’, just as they’d stopped saying the ‘aitch tee tee pee colon forward slash forward slash’ several years before. To make sure visitors got to the right place, things were set up so that if anyone just typed in the ‘naked’ domain name, without the ‘www’, they were redirected to the ‘www’ subdomain. Your website may have this perfectly valid way of working to this day. Our does: we just tell people to find us at ‘bmon.co.uk’, but if they go there, they’re automagically sent to ‘www.bmon.co.uk’.
Then people said: why don’t we do it the other way around? It’s perfectly possible to have our entire website at the ‘root domain’ level, so let’s put it there, and if people type in ‘www’ (because they wrongly assume they need to) then they’ll be redirected the other way. A short address in the browser looks neat (and it can be shortened even further for .co.uk website owners, who are now able to use just ‘.uk’ instead).
But if we’ve got a ‘www’ site, should we change it and drop the ‘www’? Technically, there’s no reason why we can’t, as long as we don’t want to use other subdomains. However, there’s no real advantage other than the aesthetic one.
Whatever you’re doing, the important thing is to ensure that everything is redirected correctly. Get technical advice on this. If it’s done correctly, the search engines will be fine with a change. So choose if you’re using ‘www’ or not, and make sure the other one redirects properly. While you’re at it, make sure that if you’re using ‘https’ that ‘http’ redirects, and if you’re switching to ‘uk’ that ‘co.uk’ redirects. In other words, all of these should redirect to the same one, whichever you’ve chosen:
Finally, use Google Search Console to tell Google what you prefer, and ensure any ‘canonical’ tags you have in your documents are updated.