Whitelisting means registration on the ICO site to participate in their sale. This requires some basic information to prove that you are a real person to invest. It is just like do KYC for new investment.
Also, the whitelist is a kind of preliminary sales list. Thanks to him, project developers can see how many people want to invest in their project and judge the popularity of their product.
Some people confuse Whitepaper and Whitelist. If Whitepaper is the project documentation (business model, technical solution, etc.) then Whitelist is simply the process when the investor has to go through all the registrations "must-do". It is the way to predict scamming and the method fo avoid fraudsters. Some projects do it to calculate the number of their investors and check their possibilities.