Oh, I see!
Let’s Encrypt is a completely automated service which doesn’t work over e-mail, but by a special computer-to-computer protocol called ACME. That means you need some kind of software application to submit the certificate request for you.
If you’re not familiar with this area but you want to go ahead with the suggestion from the hosting company, the easiest way is probably with the web site https://zerossl.com/, which is a third-party service that can take care of all of these steps inside of your web browser. (During the process, you’ll be told to make a change to your web site by posting a particular file on it to prove to Let’s Encrypt that you’re really the owner of that site.) At the end, you’ll get the files that you need to send to the hosting company.
One thing to be aware of if you do this is that Let’s Encrypt certificates are valid for only 90 days, so you’ll need to repeat this process at least once every three months. Otherwise, the certificate will expire and people start to will have trouble connecting to your site.