Minority shareholders are shareholders who have minority stakes in a company that is controlled by a majority shareholder.
The value of shares can be depressed by the existence of a majority shareholders (including a group of connected shareholders). Minority shareholders are often effectively deprived of any real say in the running of the company, and they may find that the company is run in a way which benefits the majority at their expense.
The “minority” may have more shares, but lack control due to how the company is structured: for example, they may include non-voting shareholders.
Although legal protections exist against this danger, they are not always effective. A majority shareholder cannot (at least in any country with an effective legal system) blatantly cheat the minority, but there are more subtle ways in which the majority can favour itself, for example, by preferring to deal with group companies (such as other subsidiaries of the parent).
The disadvantages to minority shareholders are also the reasons why a prospective majority shareholders are willing to pay a control premium.