A computer is a machine that follows a list of instructions called a program. An Android device is a computer and an app is a program.
An app can manipulate many types of information. These include whole numbers, numbers with fractions, and pieces of text. In the Java language these data types are named int, float, and String. Other data types are char, which is a piece of information that is a single character, and boolean, which is either true or false.
Values of different data types are stored in completely different ways inside the computer. For example, a String almost always takes much more memory (storage space) than humble int. For this reason, we have to use variables (containers) of different types to hold each type of value. An int variable cannot contain a String.