Lead scoring assigns points to leads according to rules you choose. A decision-maker in your target sector who has asked for a quote might score high, while a general enquiry from outside your area might score low. The result is a ranked list.
The score is a guide, not a verdict. It helps a small sales team spend time where it is likely to count, but a person should still use judgement. The rules need reviewing occasionally against what really turned into work, otherwise the scoring drifts away from reality.