What happens to an equipment attached to a creature that phases out?

When a permanent phases out, all Auras and Equipment that are attached to it phase out simultaneously with it. This is called "phasing out indirectly." The Equipment does not go anywhere on its own — it simply becomes tucked away alongside the creature until both return. (CR 702.26d)

When the creature phases back in at the beginning of its controller's untap step, the Equipment phases back in at the same time and is still attached to the creature, just as it was before. No "attach" trigger or spell is needed — the attachment is preserved through the phase. (CR 702.26e)

Importantly, indirectly phased-out permanents cannot be interacted with while phased out. They are treated as though they don't exist. Opponents cannot target the Equipment, and its static abilities are inactive during this time. (CR 702.26f)

Concrete example: You control a Grizzly Bears equipped with Colossus Hammer. An effect causes Grizzly Bears to phase out. Both the Bears and the Hammer phase out together. On your next untap step, both phase back in — and the Hammer is still equipped to the Bears, with no extra steps required.