A 30-60-90 day plan is a structured outline of what someone intends to focus on across their first three months in a role, broken into three 30-day phases. It shows up in two very different places: as an answer to an interview question, and as an actual working document new hires and newly promoted employees bring into their first weeks on the job.
In either case, the value of the plan is the structure itself. It turns a vague intention to "do well" into something specific enough to act on and specific enough for a manager to react to.