Infill refers to the structure that is printed inside of a closed object. If you wanted something printed hollow, you would print with a 0% infill. 100% will fill the entire object with material, causing the weight and time required to print to increase.
An infill of over 25% is not needed for most structures. This is one reason that 3D printing is useful in aviation. Drones and other similar structures can have strength yet reduced weight. In order to have a print with a clean top surface, larger prints will requrie an infill minimum of 10%.
Within certain slicing programs the user is able to create unique infill patterns in order to test the strength of each at different fill percentages. Decorative patterns such as Moroccan stars and Catfill show poor performance and should only be used if they are exposed and are part of the design. The real debate is between Linear, Diagonal and Hexagonal patterns.
Varying combinations of infill percentage and pattern can influence strength, material usage, and print times. Sometimes, in order to improve the mechanical performance of a 3D printed part, it comes at the expense of print speed, affordability, and quality.
SThrough trial and error we have found that the three most useful patterns are diagonal, linear, and hexagonal. Triangular structures can also be used to speed up the print, but when it comes to strength vs material used, those three seem to have the best results.