The 'Play' symbol indicates tape movement.
Why it is true
The arrow points to the right because, in the first reel and cassette recorders, the physical tape ran from left to right.
Source: Technology History
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.