This expression is used to describe somebody who overestimated his / her abilities or talk off the top of one’s head, exaggerate too much about things.
Example:
Tā lǎo-shì chuī-niú-pí, dà-jiā yǐ-jīng bú xìn tā le.
他老是吹牛皮,大家已经不信他了。
He is always boasting, so no one trusts him.