"¿Dónde quieres jugar al tenis?"
Translation:Where do you want to play tennis?
I had the same question and did a little web research. It's because jugar is, in the play a sport sense, intransitive (no direct object) and requires a preposition. But the English verb play is transitive, so we can omit the preposition when we translate from the Spanish. Confusingly, tocar, even in the sense to play music or an instrument, is a transitive verb.
Try working it out without looking at the tiles. But even when you do you are still learning.
With DL Spanish's word tiles, all I had to do was choose the only words available to make a sentence - no learning Spanish at all!
Actually, the website offers the word bank now as well; you have to opt out (only once, though) to be able to type in your own sentences.