It means that you must hold your original complying significant investment continuously until you apply for a permanent Significant Investor visa (for any reinvested complying investments, you are considered holding them continuously if you satisfy the reinvestment requirements). It includes the duration of your original four year provisional Significant Investor visa and any extension visas granted afterwards. You must also hold your complying premium investment continuously except any part of the investment that is a philanthropic contribution.
Visa condition 8557 will be placed on your provisional Significant Investor visa which requires you to maintain the complying investment. If you breach this condition your visa may be subject to cancellation.