I believe it means you can 'reserve your spot in the list'. It doesn't guarantee that you will get all the shares you reserve (if they are oversold, etc).
Online investments are generally not very safe. AngelList investments should be safer, because you have validation in terms of seeing investors or other people you know/may have heard of telling you they like the company/are investing as well. However, AngelList investments are still overwhelmingly like to fail, meaning you should practice good diversification.
In general, all investors will have rights to share certificates, and will get a number of shares proportional to their investment. It is up to the startup to take care of all this.