"33" stands for the tones combination third-third.
Third Tone Changes Into Second Tone
When two 3rd tones are in succession, the first 3rd tone changes into 2nd tone:
3rd + 3rd changes into 2nd + 3rd
For example, nǐ hǎo (meaning: How do you do) is actually pronounced as ní hǎo.
For the basics about tones and tones combinations, please read the following tutorial:
Lesson 1 - The Four Basic Tones