First download Etcher, then follow the instructions in Etcher to flash the moodlebox-2.6.1.img.gz file you’ve downloaded on your microSD card.
For the geeks
If you prefer to use a command line interface to flash your card: extract the moodlebox-2.6.1.img.gz file you’ve downloaded and follow the instructions to copy the disk image moodlebox-2.6.1.img to your microSD card.