A fraction is said to written using lowest terms if its numerator and denominator are relatively prime, that is, they have no common factors other than 1. (Some books use "simplest form" to mean the same thing.)
So, is written in lowest terms, since 5 and 9 have no common factors other than 1. But is not; 6 and 9 have a common factor 3.
To write in lowest terms, divide both the numerator and denominator by the greatest common factor, in this case 3:
So written in lowest terms is .
This is known as reducing fractions.