Copper Pour (also known as Copper Polygons), a very useful and widely used basic functionality of every good PCB design software.
You can use it to create a solid area on a PCB layer to lower your power tracks impedance, increase maximum current, use it as a heat sink or RF shield. You can also use it to create solid ground or power plane on multilayer board design. Those are just a few examples of why Copper Pour can be useful.
This introduction video illustrates the basics of selecting the copper pour layer, spacing rules and keepout areas.
More advanced features of Copper Pour in DesignSpark PCB design software:
Introduces spoke and isolation gap options plus the option to override settings.
Correct use of Copper Pour Outline:
Why not to delete the pour outline (up to V8, now prevented in V9), isolated islands, importance of pour outline line style.