A company’s market share in an industry is the ratio of their sales in that industry to the industry total (for a specified time period), often expressed as a percentage.
In 2019, it’s estimated that over a quarter of global shoe sales revenue were Nike. Nike’s market share for shoes in 2019 was therefore over 25%. It’s possible that Nike sold less than a quarter of the shoes sold, but the dollar value of their sales was over a quarter of the total dollar value of shoes sold. Market share may be specified for specific regions or countries as well. Market share is a useful measure of a business relative to it’s competitors, relatively independent of macroeconomic environment.
Investopedia article on market share
Wikipedia article on market share