parallel:tldr:0a7ea

This command is making use of the parallel utility to convert all JPEG files to PNG format.

Here's a breakdown of the command:

• parallel is the command that runs other commands in parallel.
• convert is the command that is executed for each file.
• {} is a placeholder that represents the input file.
• {.} is another placeholder that represents the input file without the extension.
• .png is the desired extension for the output file.
• ::: separates the command from the input files list.
• *.jpg is a glob pattern that matches all files with the .jpg extension in the current directory.

So, the command will execute the convert command for each .jpg file found in the current directory, replacing the file extension with .png in the output filename.