Mastering Strength helps categorize how strongly eMastered will adjust your audio file.
For example, a song with a large EQ boost will be boosted even further with the Mastering Strength set to high. Likewise, a song who's volume is set to "low," and also set to Low Mastering Strength will be even more subtlely affected than normal.
Different genres prefer different levels of processing, EQ, stereo width and so on. The mastering strength feature helps you pick the ideal amount of adjustment for your specific track.