Anything you easily realize with your senses, though it can be a feeling, emotion or a fact - e.g. cold, pain, need to run, shame, depression, someone's hate towards other, thrill, fear, broad stupidity your hate (the kind that makes you kill someone), that communism is bad, h (I'm from Czech republic, russian communism took us fifty years and expelled us from modern Europe), that and so on. It overflows from just knowing it to feeling it in your brain or even whole body.
Physically good song can be the one that makes you dance or feel like flying, physical stupidity takes your speech, physical inspiration is the state where you have to make it happen.