@DataJuggler assuming you had a website that you selected and you requested a certificate for it then the default configuration would be that your website is now automatically configured for https and has a certificate associated with it. If instead you optioned to manually enter domains (rather then selecting them from an existing website on your server) then the certificate will just be stored in your computers certificate store ready for you to use it.
It’s worth reading up on Let’s Encrypt in general to understand how their certificates work, but the idea behind this app is to handle the certificate request for you and to automatically apply/deploy it to your website.
Certificates from Let’s Encrypt expire after 90 days, but by default Certify will begin attempting to renew your certificate after a couple of weeks (configurable under Settings) and let you know if there is a problem doing that. If renewal is all working fine then the app will basically take care of refreshing your certificate forever until you tell it to stop.