The update to Android 4.0 Ice Cream Sandwich is rolling out from Verizon Wireless. The update is rolling out in phases. Which typically to only a few users early on in the week and expanded to a larger audience from Thursday on through the weekend.
Android 4.0 Ice Cream Sandwich adds some features such as Android Beam, Face Unlock, a new UI and improved camera (see all features and video).
In general it's always a good idea to backup your files before you start the update and have your Samsung Galaxy Tab 10.1 at least 50% charged.