song(1) is often translated as relax, loose, loosen. It is commonly used with other characters to mean lax, let go, sigh in relief, untie, free, soft, loosen a belt, etc. It can be used to describe soft, open, loamy soil.
The top of this character has 2 radicals, which means long and which means hair. The bottom also has 2 radicals. Standing together, the 2 lower radicals mean pine tree. This left side is wood, upright and stable, and the right is a phonetic element which combines the "divide" radical and the "private" radical (to mean justice, fairness, male, metric, and other stuff that I don't understand!)
In Taiji Quan, the quality of song is perhaps the most important principle to practice. Upright and rooted like a tree, soft, loose, and resilient like a hair, this is the practice. When we are song, then qi can flow freely, directed by the mind.