Chinese language vocabulary 汉语词汇 http://cn.hujiang.com/new/vocabulary/

What does it mean?

Shǔ yī shǔ èr means someone or something is extraordinarily good. Here shǔ is to count. Yī and èr means one and two. The whole phrase means if it's not the best, it's also considered as the second-best.

How to use? 

The usage of this idiom is like an adjective. For example:

（1）他的汉语水平在我们班是数一数二的。

       Tā de Hàn yǔ shuǐ pínɡ zài wǒ men bān shì shù yī shù èr de. 

       His Chinese level is extremely good in our class. 

（2）玛丽在学校里是数一数二的好学生。

       Mǎ lì zài xué xiào lǐ shì shù yī shù èr de hào xué shenɡ.

       Mary is a top class student in the school.