Here’s a random example- if we get a mix and everyone thinks the lead vocal is too quiet, we may attempt to correct that by boosting somewhere in the midrange. This may work great, but if the snare also gets turned up by this EQ adjustment, it may be a bad compromise. In this (fairly common) mastering scenario we may instead use something called a “Dynamic EQ” or “Multi-Band Dynamics”. This could allow us boost the midrange up, but have it pull back down for the milliseconds that the snare is hitting. Again, very subtle and specific.
Mastering will not ‘remix’ your music. What mastering do is to help bring that final mix to its true potential and make sure it is ready for distribution.