Differences between Solid Region vs Polygon Pour
Solid regions are just that: areas of fill (copper, on routing layers) that exist on the layer. These are useful when you definitely want solid fill (silkscreen, copper under a QFN thermal pad) that will not be changed by design rules. This still can trigger DRC violations.

On the other hand, polygon pours are "smart" in comparison. They are dynamic; you must define them and then create them. Each time you 'pour' the polygon, it is dynamically generated to satisfy clearance and other design rules, in addition to connecting to nets and pads in different ways. This is generally the best thing to use when you want to pour ground or power, but don't want to worry about specifying each place clearance is needed (almost everywhere). I use these for my 'plane' layers and to give full ground coverage on the top/bottom copper layers.