They're power-ups that are incompatible with Samus's basic Power Suit, which means she can't use them until she gets a new one in Chozodia. Obtaining the items will still activate certain obstacles, allowing Samus to remove them. The Chozo Statues that hold the Unknown Items are somewhat corroded, which could mean they were either damaged or are just very, very old.
The first item you'll find is the Plasma Beam in the Chozo Ruins. Later, you'll need to get the Space Jump in order to explore the rest of Kraid's lair. Finally, getting the Gravity Suit in Ridley's lair will allow you to return to the rest of Zebes... after you fight Ridley, of course.