"Bull in a china shop" is an English idiom that, in its most traditional sense, refers to a person who is clumsy and tends to knock things over. The meaning is derived from the fact that a bull would be expected to shatter all of the fine glasses, dishes, and other china if it were actually in such a shop. This phrase has also taken on a more metaphorical meaning that refers to clumsiness in any aspect of life. Thus, a "bull in a china shop" can be anyone who acts in a thoughtless or careless manner.
Idioms are used in the English language as a way to say something colorfully by using a culturally recognizable phrase. The words in an idiom usually come to mean something a bit different from their literal interpretation. Animals are often used in idioms, since comparing a person to an animal can have an amusing and evocative effect. One of the longest-lasting of these animal idioms, dating back to early in the 19th century, is the phrase "bull in a china shop."
To understand the meaning of this phrase it is best to imagine what would happen if the action the phrase implies literally occurred. The bull would most likely wreak havoc on all of the fine china, since it is known as an animal that lumbers about with recklessness. Much damage would likely be done, and that is the main gist of the idiom.
When describing someone in this manner, a person can be making a reference to actual, physical clumsiness. For example, a person could say, "He's like a bull in a china shop the way that he's already shattered three of those ornaments while trying to hang them on the tree." Much like the bull, the person in this sentence in causing a great disturbance through his clumsiness. The idiom makes a bit of a leap to its understood meaning, since the bull's destruction is likely not because of clumsiness but rather because of its lack of concern for the china.
It's that latter idea that gives the phrase its wider, metaphorical meaning. Much like the bull isn't concerned with the china, people can be at times careless, either knowingly or unknowingly, of the feelings or concerns of other people. As a result, the phrase has come to embody anyone who has a lack of tact. For example, someone might say, "You made her feel so upset without even realizing it; you're like a bull in a china shop."