When you go to just about any web site, your browser sets up an encrypted connection between you and the server so that anything you do on the web site can't be observed by sniffing the traffic.

Let's Encrypt is a suite of software developed by the Electronic Frontier Foundation to bring this security to anyone with a website (or anyone with a webservice, really). What Let's Encrypt provides you is a fully trusted certificate chain. As a result of making the certificate free for anyone (and I mean anyone) can use, the certificate is only valid for 90 days.

You can purchase a trusted certificate that lasts longer but renewal is so easy that unless you need a higher "reputation" cert, it's not worth it. Fun fact: cia.gov uses LE certificates!

Not sure what KanIDM is but they are probably having you use an LE certificate to create a secure connection between clients and servers. It's free and pretty easy to set up.

Optionally, if you're technically savvy, you can set up your own Certificate Authority and distribute it on your own. This gives you full control over your certificate linage but my guess is you won't get the benefits of it.