The HTTP post method is used to perform resource-specific processing of requests. This means it can actually be used in a variety of ways. Let's take a look at the first example, where post can be used to create a new resource. We'll be creating a new GitHub issue using the GitHub API.
Here, we can see our post request has included a message body and a content type to indicate what is included in our request. This is optional with post requests, and some post requests may not have any message body at all.
We can see that the GitHub server has responded with a 201 status code, indicating it has successfully created our new issue, and it's indicated the location of that issue, as well as returning a representation of that issue in the response. Post can also be used for other generic request processing. As an example, let's update the assignees of our created GitHub issue.
Again, we have a request body indicating the assignees we would like to add to our issue. We can see that yet again, the GitHub API has responded with a 200 family status code, as well a location header indicating where our issue is that has been updated. We can also see that the issue itself has been updated to include a new assignee.
Post requests are not idempotent. We can imagine that our initial request to create our issue, if retried several times, would end up creating multiple duplicate GitHub issues.
To summarize, HTTP post is used to request resource-specific processing of our request. That can be used to create new resources, or to update or make modifications to existing ones, or request other generic processing.