what do you mean by the 'lowest' boiling point and the 'highest' boiling point of a liquid? Does that mean if a liquid has the highest boiling point then it will evaporate first during fractional distillation?

It means that  the liquid which has the highest boiling point condenses first and is collected in the lowest compartment and the fraction with lowest boiling point is collected at the top of Fractional distillation column

For example : during fractional distillation of crude petroleum - Petroleum gas is coollected at the top as it has least boiling point of 40 ⁰ C