The root signature needs to declare all data your shaders expect. If your shaders don't expect any data, you can simply use a blank root signature. The root signature is simply like a parameter list (function signature) for your shader pipeline:

RenderTarget ExecutePipeline(RootSignature sig); // obviously this is not a real representation

It's just saying, if your shaders expect 14 cbv's, you're root signature needs to represent that.

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all descriptor heap entries covered by descriptor tables in the root signature must be populated with descriptors by the time the shader executes

this is saying that all the descriptors that a descriptor table points to (the table which the root signature is pointing to), MUST exist before your shader runs

Nope, you don't need to fill out anything that your not using

For Tier 2 your entire shader-visible UAV and CBV descriptor tables need to have valid descriptors in them. So if your root signature specifies a descriptor table with 8 UAV's and your shader only uses 4, then there still needs to be 8 valid UAV descriptors in the table that you specify with SetGraphicsRootDescriptorTable. NULL descriptors count as "valid" in this case, so you can fill up the rest of your table with NULL descriptors if you want. 

The validation layer will complain at you if you mess this up, but unfortunately it will only do this if you run on Tier 2 hardware.  There's not a whole lot of DX12-capable Tier 2 hardware out there ever since Nvidia upgraded their Maxwell and Pascal GPU's to Tier 3.