Everything the platform supports: the container, the growing medium, the plant and the water they hold. It measures that total load, not the plant on its own.
Taring removes a reference load, such as the empty container, from the reading. It does not isolate plant biomass automatically.
So a rise after watering is water, not growth. A fall can come from evaporation, from transpiration (water leaving through the leaves) and from drainage. Rain, harvesting, handling the pot and changes to the setup also change the reading.
The platform shows what changed, not why. It does not measure nutrient absorption, root uptake, biomass growth or yield.