Tencha is the name for the green tea flakes used to make matcha powder. You may have heard that matcha is shaded for four weeks before harvesting. After the leaves are harvested, they are then steamed and dried without rolling (rolling, in addition to shaping the leaves, breaks down cell membranes to transform the tea leaves into sencha or gyokuro). The unrolled leaves break apart creating flakes. As a flake, the leaves are then easier to grind into smaller sized powder grains.
As a tea leaf, it is actually more difficult to steep due to the lack of rolling, and Obubu recommends that you steep it as a cold tea.