It means that the substance contains as many water molecules as it has SiO2 molecules. But the water molecules are not covalently or ionically bonded to the SiO2. Instead, they are kept together by hydrogen bonds. read more
H2O is water. The H means hydrogen. The 2 indicates that one molecule of water contains 2 hydrogen atoms. The O indicates oxygen. There is no number for oxygen present, so there is just one oxygen atom in a water molecule. That's why it's H20. Water can also be known as dihydrogen monoxide. Those prefixes just mean the numbers. read more
By writing $\ce{AB.xCD}$ chemists mean that there are CDs are found in the crystalline framework of AB. The most common example of this is water trapped inside the crystal structure of ionic compounds. read more