Berkshire Hathaway is a public, stock company. People can buy and sell shares in the company easily. Most public (stock) companies have a turnover of shareholders, i.e. many people buy and sell shares on speculation or for a time. They are ready to buy or sell when there is good or bad news.
Berkshire Hathaway has a different type of corporate culture, and it attracts shareholders who hold onto (retain/keep) their shares for a long time, in good times and bad. So, Mr Buffet is calling these shareholders 'high grade', i.e. high quality shareholders, and that, in comparison to other companies, on average only 20% Bershire's shares are sold and bought each year. 80% of the shares are not traded, but kept.