'to frank' is to 'put a stamp mark' on something which for postal purposes meant postage had been paid but for the context here it means that tax had been paid on the dividend at the company level.

So, if the company tax rate is 28% and the personal tax bracket for a certain level is 15%, the investor can claim the difference back from the tax authorities. The rules on dividend franking varies from country to country. The purpose on this sort of with-holding tax at the company level instead of at the stock holder level is to prevent tax evasion. So, if contry A does not have double-taxation agreement with country B, and dividend is received from coutry B, the investor would have to declare the dividend and pay tax in country A on top of the tax in country B, etc.

Certain countries exempt taxation on foreign dividends and Singapore is moving towards abolishing dividend tax totally as company profit had already been taxed.