Most people just see this as what is displayed in a browser, although some browsers do hide this but it stands for Hyper Text Transfer Protocol and if you see an S on the end then that means the communication is encrypted. There is a huge amount that can be said and indeed has been written about HTTP, so there is no value in my repeating that, so I will defer to Mozilla and recommend reading HTTP | MDN.
One area I keep having to try and remember is HTTP Response Codes. When any software sends a request to a server via HTTP the server will respond and include a response code. They are categorised as follows:
For a complete list and full explanation see HTTP response status codes - HTTP | MDN.
The current, predominant standard is HTTP 1.1, there is also HTTP/2, which is designed to reduce latency by changing how the data is packaged and transported, it does not change the status codes, header fields and so on. Then there is also HTTP/3, which is more different, in that it is based on QUIC, unlike the previous versions which are based on TCP. It is also worth noting that all of these can, and do, co-exist within a browser and various support libraries.
A lot of HTTP traffic is browser based, however there is a lot of HTTP traffic that does not have a browser at one end and this is where we have more control over what happens but also less in the way of standards, or rather there are several. I have also seen organisations adopt non-standard approaches as their standard for their own reasons.
See also REST