The document discusses mathematical expressions and how to combine them. It defines an expression as a calculation procedure written with numbers, variables, and operations. Expressions have terms, with the x-term being the term with a variable and the number term being the constant term. To combine expressions, like terms are combined in the same way numbers are combined. For example, 2x + 3x = 5x and -3x - 5x = -8x. However, unlike terms like x-terms and number terms cannot be combined since they are different types of terms. The overall expression after combining all like terms is called the simplified form.