The infill is the internal structure of the model, it is an important part of the part and can effect the overall quality of the print.
Please note that possible infill settings should be checked in the slicer itself, not referencing this image.
Infill density is necessary for internally supporting your print, and as such the minimum density should be 10%. At 0% you would have a hollow object, at 100% you would have a solid model. Infill density should, at most, be 50% and to put that into perspective take a look at the picture below, if you feel that your print needs more than 50%, it is likely that 3D printing is not the correct method of manufacturing for your part.
Infill patterns alter the properties of your infill, and can give your parts additional strength in certain directions, while also making it weak in other directions. Cubic is used as the default as it provides strength in all directions and is easy to print.
Additional info can be found here.
