The tape is magnetic. As Lauren said, but maybe in more laymans terms, live sound (audio) is first recorded, then converted into an electrical audio signal. The tape records these signals as magnetic information, this is happening on a very small scale relative to a human. The tape player will read that information and play that audio that is there.
Here is an article that explains, but if you know no physics or basic audio principles this can be a hard concept to get: https://reverb.com/news/how-does-magnetic-tape-work-the-basics
Records are the best example of audio at work. If you play a record player with no speakers attached, you can hear the song playing quietly at the point of the needle. If you have access to this try it, I'm sure you can also look it up on youtube.