Any series of characters — letters, numbers, punctuation marks, and blanks — is called a string of characters. The number of characters in the string is called the length of the string. The string may consist of words or of randomly chosen characters. It can also be a single character, or even no characters at all. The latter is called the empty string, and is the only string of length zero.
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. A variable is a container inside the device. Many variables hold numbers, but one type of variable, called a string variable, is capable of holding a string.