In mathematics, when we see the = sign, we have an equation. In an equation,we mean that the expression on the left hand side equals the expression on the right hand side. The equation, means that there is a value for that makes the equation true (or false). Mathematical equations can always (5x + 2x = 7x), sometimes (y + 3 = 5), or never (2z + z = 3z + 1) be true.
An equation is like a balance. If the objects on the left balance with the objects on the right side ( if a = b), then to maintain the balance, whatever you do on one side you also do to the other side.