The document discusses equations and their properties. It defines an equation as a mathematical statement asserting the equality of two expressions, with unknown quantities denoted by letters at the end of the alphabet and known quantities by letters at the beginning. Equations can express relationships between knowns and unknowns, and solving an equation means expressing the unknowns in terms of the knowns. Types of equations include algebraic equations, which involve only algebraic expressions in unknowns, and linear equations, which are algebraic equations of degree one.