Combine information from two different nonfiction sources on the same topic into one organized set of notes, without simply copying one source after the other.
Real research almost never comes from a single source. Synthesizing means reading more than one source on the same topic and combining what you learn into your OWN organized notes -- grouped by subtopic, not just listed source by source. Done well, synthesis notes read as though one person understood the whole topic, not as two separate summaries stapled together.
Consider two short sources on volcanoes. Source A explains general causes: "Volcanoes form where molten rock, called magma, forces its way up through weak points in Earth's crust. Pressure builds until an eruption releases gas, ash, and lava." Source B describes a specific case: "In 79 AD, Mount Vesuvius erupted with such force that it buried the nearby Roman city of Pompeii under volcanic ash, preserving much of the city -- including its buildings, art, and even the shapes of its residents -- in remarkable detail."
Mastery looks like: They can combine two nonfiction sources into organized notes grouped by subtopic, including at least one sentence that genuinely connects information across both sources.
Common stumbles: Summarizing each source separately, one after another, without writing a sentence that actually connects information from both sources together.