What it means
I'm vegetarian. 素 means plain, meat-free — a Buddhist term that became everyday vocabulary.
When to use it
For vegans, add 我不吃鸡蛋和牛奶 (I don't eat eggs or milk). Watch out for oyster sauce and lard, which aren't considered "meat" by everyone.
Examples
wǒ我chī吃sù素，yǒu有sù素de的cài菜ma吗？
I'm vegetarian, do you have vegetarian dishes?
wǒ我chī吃sù素，bú不yào要fàng放ròu肉。
I'm vegetarian, please don't add meat.
tā她chī吃sù素hěn很duō多nián年le了。
She's been vegetarian for many years.
The full set of survival phrases for greetings, dining, shopping, and getting around, each with audio.