Hypertext Transfer Protocol, is a protocol used to format data in a specific syntax which is transferred across internet. Nearly all content that we consume through internet follows HTTP.
All requests and responses following HTTP protocol are transferred in raw format. Therefore, any malicious actor can intercept this communication.
It is the secured version of HTTP. The 's' here literally stands for secure, it uses TLS/SSL to encrypt the HTTP requests and responses. i.e. data is no longer transferred in raw format.
SSL uses public key cryptography with 2 keys, public & private. Client receives public key via SSL certificate. Client & Server create session keys to enrypt all requests & responses.