To move a server running on HTTP to HTTPS, the first thing we have to do is to get the SSL certificate, which may be either self-signed or a certificate signed by a trusted certificate authority such as Comodo, Symantec, or GoDaddy.
To get the SSL certificate signed by a trusted certificate authority, we have to provide them with a Certificate Signing Request (CSR), which mainly consists of the public key of a key pair and some additional information, whereas a self-signed certificate is a certificate that you can issue to yourself, signed with its own private key.
Self-signed certificates can be used to encrypt data as well as CA-signed certificates, but the users will be displayed with a warning that says that the certificate is not trusted by their computer or browser. Therefore, you should not use them for the production...
