Infill is a value usually represented in percentage that shows how much a solid model should be filled in with material when printed. Normally, unless you want maximum strength, you would not need to go for maximum fill of a 3D model, especially if you also want to save on material costs, model weight and want to get the print faster. Going at up to about 20-30% percent Infill is usually more than enough for normal use, unless you want something specific like high fill percentage for optimal strength. When you are not using the maximum fill percentage a special pattern is used to create strong and durable inside support structure.