There is no direct English translation for Song, so the words "release" and "relax" are frequently used. "Relax" is complicated, as there are many different models or definitions in the taiji world. In the most simplistic sense, you cultivate the ability to let go, release muscle tension while maintaining structure. If you tell people to relax, they'll often collapse their structure and become human spaghetti. Song is the principle of releasing and relaxing while maintaining structure, so bones up / flesh down. Structure and alignment are maintained while releasing or letting go of muscle tension.
There is a mental component as well. You have to develop awareness of muscle tension before you can let it go. As you learn to stop gripping your abs, you will have to be aware of the gripping and then figure out what you need to do to release those muscles. That's a mental exercise as well as a physical one. You will likely be repeating it countless times a day. It can take a long time to learn to release habitual muscle tension. It tends to sneak back in.
Here's a description of the six levels of Song in the context Yang style taiji:
https://discovertaiji.com/en/blog/six-levels-of-song