Infill is simply a repetitive structure used to take up space inside an otherwise empty 3D print. For the majority of prints, infill is hidden from view by shells, top, and bottom layers.

What’s inside the 3D entity is called infill. As you can see in the picture above, it’s a grill and it’s yellow.
Fill Density can be changed, according to the structural needs of the object. The range goes from 0 to 100% infill density.
0% infill means the object is empty inside; 40% means it has a grill or another type of pattern; 100% infill means the object is completely full, it’s solid.
Most of the times, the infill structure is a Recti or are just parallel lines, but it can also have a more aesthetic pattern, to show off.
The higher the infill is, the more the object will weight. A higher infill density makes for a heavier, more solid print. In contrast, a lower infill density would provide a simpler, more lightweight result. Infill density can also affect a print’s strength, buoyancy, and material used.
The Infill can be used as part of the object design, for example, a cleaner has a central grill. This grill can be the infill. By deactivating the top layers, the grill pattern shows.
Now, let’s take the following example:
Use the following prin setting, then click on the three-point button.

When you deactivate the top and bottom infill solid, the grill shows. This way, the top, and bottom layers don’t cover the object and you can see through.