Just different classes of shares, like first class and coach on an airplane. The B shares don’t trade, but they are convertible to A shares which do trade on the public markets. Typical split share structures give super-majority voting rights to one class, and sometimes other benefits (like dividends). Many (though a very small percentage of traded companies) public companies in the US (especially tech, but others too) have split share structures so founders can retain voting control, although generally investors don’t like these structures because they can’t effectively vote out management or board members, and founders can block mergers that might be beneficial to other shareholders.