Two algebraic expressions are equivalent if they always lead to the same result when you evaluate them, no matter what values you substitute for the variables. For example, if you substitute x := 3 in x + x + 4, then you get 3 + 3 + 4, which works out to 10; and if you substitute it in 2x + 4, then you get 2(3) + 4, which also works out to 10.
