Comparing two rasters using ArcGIS Raster Calculator

assuming that both rasters reflect the same variable on different dates and value scales, for example NDVI start date (NDVI-i) - NDVI end date (NDVI-f). Assuming you performed the operation NDVI-f - NDVI-i:

1. Positive values indicate a recovery of the vegetation cover at the end date.
2. Negative values: a loss of coverage between the initial and final date.
3. Value 0: this zone remained unchanged.

I understand that ArcGis has tools for detecting changes, so you might want to look into it.

Another aspect, instead of working with the change of signs, negative or positive. It is advisable to define a threshold of change, ie, what range you consider a significant change. For example, for a scale of 0 to 1, you could decide as threshold value 0.03, values below this range are not considered a change.