As pointed out already, you do not need to worry about lining up the polygon with the board outline. Polygon pours will be automatically clipped to the outline assuming that the Orphans setting is disabled.
If Orphans on the polygon is enabled, you simply have to get the polygon close enough to the board outline that the board edge to copper clearance setting in the DRC is large enough to remove any trace of the polygon
In terms of your errors (these are errors, not warnings!), they are coming from the fact you are using the wrong layers for things.
The DRC indicates the errors are coming from Layer 1, which indicates that you have put your measurements (dimension arrows (*)) on the Top copper layer (1). You should never do this.
If you put measurements on a copper layer, especially inside the board outline, the measurements will appear in the final Gerber output as copper. This is why the DRC generates errors.
All measurements should be placed on the "Measures" layer (47). This is a layer which is ignored by the DRC and will ensure you don't get any errors related to them. It is safe to use this layer for this purpose as it won't typically produce any physical output
Note (*): There is some confusion between the name "Dimension" which is the name of the tool to draw measurements, and the Dimension layer (20). These are actually two completely different things.
The Dimension layer is where you draw your board outline. The "Dimension" tool is used to draw measurements.
This is basically a really bad naming choice for the dimension tool when it was first introduced. Ho hum.
Best Answer
A polygon pour is use to make, typically larger than a trace, arbitrary shapes in copper.
A power plane is usually an internal layer, that is "solid" copper. Typically it is used as either the ground return and/or the power rail. Usually there will be two power planes, one for ground return and another for the power rail.
I use the word typically and usually because, it's just metal, so you can do whatever you want with it. Split planes are an example of this. Also, I've used both polygon pours and split planes to route motor return currents separate from the other copper planes on the PCB.
Per the comments: A large area of copper can be used for several things: