An in-fill grass is a grass that has a material added to the top to fill in the area between the blades of grass and weigh it down. If the pile height is particularly long then the in-fill can help the blades of grass remain vertical. In domestic lawns this is usually sand, but it is also common in sports pitches where the grass tends to be slightly longer. The type of infill can sometimes affect the colour of your grass, for example it is common in modern sports pitches to use black rubber infill which can affect the overall impression of the surface. In domestic lawns it is not usually visible unless it has not been properly spread over the grass.
A non in-fill grass is a grass that does not have a material added to the top, usually because there is a realistic thatch present in the grass, there is a higher stitch rate, and because it is secured by pins. Non in-fill grasses save on a maintenance costs because there is no in-fill to replace. Sand is still present in a Trulawn installation, but it is part of the groundwork that creates a stable base and effective water drainage. Non in-fill grasses are suited to homes with children and pets as there is no chance of carrying any in-fill indoors.