The validity of a given visa represents the dates that the visa is effective. For instance, if a visa for Turkey is valid for 90 days, you must travel to Turkey before that period of 90 days ends.
Maximum stay is the time that you have been allowed to stay in the country. The stay will begin on the date you initially enter your destination.. Using Turkey as an example, the default maximum stay is 30 days, which means you are allowed to stay in Turkey for 30 days within the 90 days that your visa is a valid.
Please note that all countries have unique requirements and interpretations of visa validity and maximum stay.