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The transition region (or solar transition region) is a relatively thin (on the order of 100 km) layer of the Sun's atmosphere constituting the transition from the chromosphere to the corona above. The term was coined for the layer in which the temperature is changing substantially with distance from the Sun, from that of the chromosphere (on the order of 20,000 K) to that of the lower edge of the corona (hundreds of thousands to a million K). The reason for the very high temperature gradient is part of the mystery of the corona's extreme temperature: various active magnetic-field phenomena are plausible contributers but it is still an area of high research interest. Given the transition region's temperature-range, the ionization of various resident elements (which depend upon temperature) allows study of the transition region's temperature and constituents, and affects the spectral energy distribution of the Sun, affecting both absorption lines and emission lines. Overall, the transition region is optically thin and we see the photosphere through it.
It is presumed stars have a transition region, but the Sun's offers by far the best study and observation opportunities.