fzf is an interactive tool for fuzzy-finding and will take input from stdin. For example, `ls | fzf` will allow you to search through a list of files in your working directory. It also has an optional preview pane that can be used to show the output of a command.
In this case, the empty string is provided as input because we're not interested in selecting options. You can then type your jq filter in fzf's query box and the command will run against filename.json in the preview pane on every kepress (the {q} is your currently typed query).
In this case, the empty string is provided as input because we're not interested in selecting options. You can then type your jq filter in fzf's query box and the command will run against filename.json in the preview pane on every kepress (the {q} is your currently typed query).