Checks the resolution of particular image types (color or grayscale images, 1-bit images or any images) against a particular value, e.g. checks if the resolution is not more/less/... than x ppi.
This Action checks for the effective resolution; if the image has been resized, this is the resolution after scaling.
As of PitStop 2018 you can disregard any soft-mask images that are used in the PDF, by selecting the Ignore soft-masks option. The resolution of a soft-mask image can be lower than what it acceptable for images, but as this does not affect the end result, it's safe to exclude them from the resolution check.
Note that the log message also mentions the resolution of the matching images.