One of the mechanisms to keep Internet traffic safe is to use encryption - you can see this working if you visit a website and you see the padlock icon in your browser (it can be in a variety of places depending on which browser you're using). The encryption is possible through the use of what's called an SSL (secure sockets layer) certificate which is issued by a Certificate Authority (CA).
The CA has the responsibility of making sure the website who is presenting you with its security certificate is legitimate - there are checking processes which are supposed to be followed, and for critical sites (eg banking, high profile online shopping, government, defence etc) there are a LOT of hoops to jump through to get their certificates, which means they cost more.
There are also cheaper certificates available, even free ones, which to the general Internet user can look identical to the expensive ones, but they can come with a few catches.
One of the largest free to use certification authorities, Let's Encrypt, has just had one of their root certificates (think of it like the foundation of their certificate services) expire today. What this means is if a website has not been kept up to date, and is using a certificate which relies on this root certificate, anyone going to visit this site will be presented with a warning message that the site they are visiting may not be safe - similar to the below images.
This demonstrates the importance of keeping a website up to date and healthy. Creating a website and posting it to the world, then sitting back and leaving it alone, can lead to problems like this and undoes all the great work you've done to get yourself a nice website.
If people visiting your site get a message reporting it can't be trusted, that could lead them to go elsewhere and you then lose those business opportunities.
If your website has been affected by this and you're not sure what to do, reach out to the team at Calvert Technologies and we'll see what can be done to get you back online and in business.