How is the process of installing the letsencrypt.org certificate on Linux on Apache servers?

I really enjoyed the idea of using letsencrypt.org certificate. I found several tutorials on the internet, including this on Digital Ocean.

All the steps worked correctly, but as I do not speak much English I would like more detail on these steps. Among the questions are:

• Where are certificates obtained?
• Why do I need to enter an email?
• What is the difference between chain.pem and fullchain.pem ?

My goal is to gain a deeper understanding of this process.