Upsampling and downsampling define the method used for interpolation of the data on non-natural scales. E.g. resolution of Sentinel-2 data (R,G,B and NIR bands) is 10 meters but in some occasions you would want to look at the data with higher scale (e.g. at 1 m pixel resolution) or lower scale (e.g. 1000 m pixel resolution).
A default option is "nearest neighbour", which is best for performance. "Bicubic" is often nicer on higher scales.
Note that the data are always exactly the same - it is just the interpolation method.
An example of the NDVI image of the field: