The proposal is to enhance the taxation of investment income in a corporation by eliminating the refundable element of the corporate investment tax. 

Basically, when a corporation earns business income it will pay a low rate of tax, so it will start out with more to invest than would a shareholder if the income was issued as a dividend and invested. When the corporation earns investment income, however, this income is taxed at a special high rate (not the regular corporate rate) that is designed to be the same tax rate that an individual would pay on the same investment income. 

The way the law works currently is that a part of this special high tax is refunded when a corporation pays dividends to shareholders. The proposed change is to eliminate the refund, so this income will essentially be taxed twice (once at the high corporate rate and then again as a dividend in the shareholder's hands) which should mimic what would have happened if the shareholder invested it personally.