I didn't understand it well, but I finally understood the meaning, so I will describe it as an output.
The image above describes the code to display the form for posting a comment from the detail screen of the tweet. I couldn't understand why I used model: @tweet and @comment following form_with, but I solved it, so I will explain the reason below.
In this case, it is a situation where you comment on a tweet, and since the comment is linked to one tweet, you can not specify the comment alone as the destination of the form, so it is linked to @tweet, @comment The reason is that the destination is specified as a set with.
The description itself @tweet, @comment specifies routing. What does that mean? Look at the image below.
In this case, I want to create a comment, so the action I want to activate is the create action of the comments controller. And if you look at the prefix item on the far left, you will see a path called tweet_comments. The bottom line is that you have to specify this routing if you want to submit a form and make a comment.
The second reason is that model: [@tweet, @ comment] specifies that routing.