xdg-mime:tldr:9812f

This command is used to retrieve the default application associated with a specific file type.

Let's break it down step by step:

1. xdg-mime query default: This is the base command that we are using to query the default application.
2. $(...): This is a command substitution syntax in bash. It allows the output of a command to be used as an input for another command.
3. xdg-mime query filetype ${filename}: This command is used to determine the file type of a given file using the xdg-mime utility. ${filename} is a variable holding the name of the file for which we want to find the default application.
4. Overall, it retrieves the file type of the given file using the xdg-mime query filetype command, and then uses the obtained file type as an argument to the xdg-mime query default command to determine the associated default application.

In simpler terms, it determines the default application for a particular file by querying the file type first and then obtaining the default application associated with that file type.