You could call it the monthly version of the solar return — run on the Moon instead of the Sun. The Moon comes back to its position in your birth chart roughly every 27.3 days, and that moment is used to cast a new chart showing the emotional, instinctive weather of the month ahead.
Your birth chart is fixed and the solar return comes once a year, but emotional life fluctuates far more often. The Moon's ~27.3-day cycle (this is the sidereal month, measured against the stars — a different measurement from the 29.5-day phase cycle on the calendar) offers a natural unit for tracking monthly shifts in mood.
So a lunar return is not used for long-term themes; it answers "how will I feel this month, where will my attention go".
All three "return" techniques work on the same principle, only at different speeds: a solar return once a year, a lunar return once a month, a Saturn return once every ~29.5 years. The slower the planet, the rarer — and the deeper — the return.
The yearly version of the same technique, run on the Sun.
Comparing the planets' current positions against your birth chart.