market share is the portion of the consumer base, in a given industry, that is controlled by any one of the competitive companies in that industry.
For example. 100,000 people buy toilet paper, amounting to 100,000 dollors in total market size. There are only 2 companies that sell toilet paper, brand A, and brand X.
If 75% of the 100,000 people buy brand X, then Brand X have made 75,000 dollors and have achieved 75%market share, and Brand A has only 25%