ag:tldr:f298d

This command is using the ag command-line tool to search for a specific string or pattern in files.

Here's a breakdown of the command:

• ag is the command to invoke the ag (The Silver Searcher) tool. It is an alternative to the commonly used grep command, but optimized for faster searches.
• -l is an option for ag which stands for "list files matching the pattern." It tells ag to only display the names of the files that contain the specified string or pattern.
• ${foo} is a variable enclosed in curly braces ${}. The value of the variable foo will be substituted in place of ${foo}. This assumes that foo is a shell variable defined elsewhere in the script or environment.

So if you run this command in a shell, it will search for the value stored in the variable foo within files and print the names of the files that contain that value or pattern.