TechNestA visitor looks for the lock because browsers warn when a page is opened without it. The certificate binds a name to a key for a term. It tells the browser that this connection matches that name. It does not say the shop is honest, the price is fair, or the company exists.
The name has to match
A certificate for example.com does not cover shop.example.com unless the certificate says so. Order the name you will actually publish. If the site will use www and the bare name, both have to be on the certificate or one has to redirect before the lock matters.
The term is on the card
When the term ends, the warning returns even if the site files are fine. The date belongs next to the domain's date so both are renewed on purpose. Certificates are a shelf of their own. The domain they bind to starts at domains.