What is a "prosumer," and why is it the pivotal actor in a modern local energy market?

A prosumer both produces and consumes electricity — e.g. a household with solar panels — so it is a primary production unit, not just a load.

In the old market a household was purely a consumer: it drew power and paid for it. A prosumer owns distributed generation (rooftop solar, a small battery) and so both generates and uses energy. That single change is what drives the shift to peer-to-peer trading: production is now spread across thousands of small endpoints instead of a few big plants, and each prosumer can sell its surplus to neighbours. Prosumers are the main production units of a modern local energy market — which is also why their weather-dependent output is the main source of instability.

Go deeper:

• Prosumer (Wikipedia) — The energy-sector sense: households that feed surplus generation back into the grid.