It's a very common phrase in Mandarin Chinese, used to express your identity or state of being.
For example:
* Wo shi yi ge xuesheng. (我是一个学生。) - I am a student.
* Wo shi zhongguo ren. (我是中国人。) - I am Chinese.
* Wo shi hen kuai le. (我很快乐。) - I am very happy.