It is the proportion of sales figures ( Usually in percentage) achieved by an organization over a specific period of time.
For example: A care manufacturer has sold 25% car in all sold cars ( out of 100% ) then market share of that company will be called as 25%. It means that if 100 care is selling then 25 cars of that particular brand is getting sold.
It is one of the measures to evaluate the ccorporate performance of a brand over specific time frame. Companies might pose serious competition to each others to achieve no. 1 position .