This is the alway repeating type of sentence "To je...", "To jsou....", "Jsou to...", "Je to...". You will meet it iver and over and over. And you will find people asking in the respective discussions over and over and over. So you can look into those other discussions for explanations as well.
We imply used "To" here to refer to something indeterminate, both singular and plural.
Thank you for your reply. I do read many discussions without always commenting. This one was relevant, but still incomplete. https://forum.duolingo.com/comment/24800541 And I try to utilise the tips and notes. As a beginner in Czech I have few other resources for usage, though. I like to use dictionaries with usage examples. With the Duo learning system, you can often push ahead, without full understanding. Now I am reviewing the early sections, as one must do in any language learning. Finally, with respect to your last sentence, this structure is familiar in other languages.