The phrase "spin axis" refers to an imaginary line around which an object, like a spinning top or planet, rotates. Think of it like a stick going through the middle of the object, allowing it to spin smoothly.
There can be irregularities, of course, such as the odd spin axis of Uranus, which for as - yet - unexplained reasons is nearly tilted on its side, and Pluto's odd orbit, which occasionally takes it inside the path of Neptune. (sciencemag.org)
Climate oscillations evident in Fig. 4 of Hansen et al. [52] were instigated by perturbations of Earth's orbit and spin axis tilt relative to the orbital plane, which alter the geographical and seasonal distribution of sunlight on Earth [58]. (journals.plos.org)
Mercury's orbit around the Sun exhibits a slow precession (spin axis direction changes in a circular motion). (newworldencyclopedia.org)