Move the slider to the right to simplify the geometry of the model. This will reduce the number of faces while preserving the shape of the original object.
By default, the slider is not linear. The middle of the slider is 75%. This is useful because we often need high simplification value, so this helps by giving more control when setting the slider close to 100%. You can make it linear by clicking on the curve icon on the left of the slider. This won't change the simplification level itself, but only how the slider is displayed.
Alternatively, you can enter a precise value in the input field on the right.
Mesh Simplification works best on closed/manifold objects. The algorithm works on groups of faces touching each other. It may not yield good results on models made of many small parts. For instance, a tree leaf made of two triangles will only be simplified as one triangle at best, or disappear completely.
Enabling this will avoid simplifying faces that are near UV seams. Use this if you see texture artefacts or extreme deformation.
This applies additional constraints so that the shape of the simplified model is as close as possible to the original one. However, the simplification will not be as efficient.