PSOs, Shaders, and Signatures

In order to render using DirectX 12, the complete description of the render state needs to be captured in a ID3D12PipelineState interface object (PSO). Compiled shaders for all stages you use are bound to the PSO at creation time. In order to share data between the HLSL shader and the CPU, the ID3D12RootSignature interface object describes how the shader expects parameters to be bound and is also part of the PSO.

Further reading

MSDN: Root Signatures