Instead of defining joy, the speaker gives it four quick images, each one a different way of picturing it. The first, a swan in the mind, suggests something graceful and pale that glides in quietly and makes the inside of your head feel calm and beautiful. The second, a door open to anything, shifts the feeling to possibility: joy is the sense that nothing is shut off and whatever comes next is welcome.
The last two images are stranger and more interesting. A "psalm against money" sets joy against buying and owning, as if it were a song of praise that can't be bought or sold. Then comes the night fox, a wild, secretive animal that appears briefly in the dark and is gone. That suggests joy can't be summoned or kept, only noticed when it shows up.
The mood is light, free, and a little mysterious. The poem is so short that it feels like a flash of joy itself, and the lack of punctuation lets the images drift past without being pinned down.