A drag along right allows a majority shareholder (ie usually a shareholder holding more than 50% of shares in a company that have voting rights attached) of a company to force the remaining minority shareholders (ie usually a shareholder holding less than 50% of shares in a company that have voting rights attached) to accept an offer from a third party to purchase the whole company.
The majority shareholder who is 'dragging' the other shareholders must offer the minority shareholders the same price, terms and conditions that the majority shareholder has been offered. For example, a majority shareholder who holds 75% of the shares in the company who agrees to sell their shares in a share sale to a potential buyer, must offer the same price for the shares to the minority shareholders if they want to 'drag them along'.
A drag along clause will allow the majority shareholder to 'drag' the remaining minority shareholders with them and require them to sell their shares to the potential buyer at the same price, in order to allow the buyer to purchase the entire company.