mm2gv:tldr:3a9dd

This command consists of several parts that are connected by the pipe symbol "|". Let's break it down:

1. cat ${input-mm}: The cat command is used to concatenate and display the content of files. In this case, it is used to display the content of the file named ${input-mm}. The ${input-mm} is a placeholder that represents a variable which holds the name of the input file.

2. |: This is the pipe symbol, which is used to redirect the output of one command to be the input of another command.

3. mm2gv: This is a command (presumably a custom one) that takes the input from the previous command and performs some transformation or conversion. It likely converts a file in the .mm format to a Graphviz .gv format.

4. > ${output-gv}: This symbol > is called the output redirection operator. It is used to redirect the output of the preceding command to a file named ${output-gv}. The ${output-gv} is another placeholder for a variable that holds the desired name of the output file.

In summary, this command reads the content of a given file (${input-mm}), passes it to a custom command (mm2gv) for conversion, and then saves the converted content into another file (${output-gv}). The actual file names would be provided by substituting the placeholder variables with specific values.