250 means ‘moron, stupid, foolish, etc.’ A rather mild tone, not a cursing word.
It is pronounced as ‘二百五 er4 bai3 wu3’ rather than ‘二百五十 er4 bai3 wu3 shi2’.
Many are often used on social media.
666 – Awesome
23333 (could be more 3) – similar to LOL
520 – I love you (similar sound of 我爱你 in Chinese)
1314 – All my life/Till eternity (similar sound of 一生一世 in Chinese)
7788 – almost (差不多)
Chinese Tuition Singapore 
That’s why Chinese students everywhere (China, Taiwan, Singapore, Malaysia and America-born Chinese, etc) are good in Math.