Few poems start with something as ordinary as urinating, and that's part of the fun. The speaker is simply relieving themselves on a rainy night, and the whole poem is one exclamation of delight at how good and unrestricted that feels. The title, "Micturition," is a formal, clinical word for peeing, and it sits in a funny contrast with the plain, bodily act the three short lines describe.
The word "freedom" is what lifts the moment. It suggests more than physical relief: a sense of being unobserved, unhurried and at ease in the dark, with nothing to perform or hold back. The rain adds to this. It suggests the speaker is outdoors, or at least listening to the weather, and the body's small act seems to join in with the larger natural one. The mood is quiet, contented and a little cheeky. The poem finds something almost joyful in a private moment most people wouldn't think to mention.