Infill is, briefly explained, the amount of material used in between two walls. Infill only applies to FDM prints, where simple plastics like PLA, ABS, PETG, and many more are used.
It plays an important role in a part's strength, structure, and weight.
You can find all basic information in this article here:
Also interesting to know
If you are looking for high stability, you might not notice a big difference between 80% and 100% infill. The difference is simply too small. Since 100% infill can often compromise your design, we offer the option of 95% infill as this is a more optimal setting for an (almost) solid part.