convert:tldr:6d877

The command you provided is using the "convert" command in a Unix-like operating system.

Here's a breakdown of the command:

1. "convert": This is the name of the command-line utility used to manipulate images. It is part of the ImageMagick software suite, commonly used for image processing.

2. "${image1-png} ${image2-png} ${image3-png}": These are variables enclosed in curly braces that represent the file names of three input images. Each filename has the extension ".png". The specific names and locations of these files are not specified, so you would need to replace these variables with actual file paths and names when running the command.

3. "-append": This option is used to vertically append the input images one after another to generate a single output image. The images are appended in the order they are provided.

4. "${image123-png}": This is a variable representing the output file name. Similar to the input images, you would need to specify the actual file path and name when using this command.

So, all in all, this command takes three input images (image1.png, image2.png, and image3.png), appends them vertically, and generates a single output image named image123.png.