Why it matters
A CNAME lets www.example.com follow example.com without a second address to maintain. It is common when a domain is connected to a website builder or a marketplace sales page. The key limit is that a CNAME cannot sit beside other records at the same name. That is why most providers do not allow one at the bare domain, where mail and other records already exist. A common mistake is adding a CNAME at the root and breaking email. Check the record type your provider asks for, because some use special alias records instead.
Source: datatracker.ietf.org