What is the meaning of "Caucasoid" and "Mongoloid"? What are the reasons for different bone structures?
Those are two of the three recognized major racial profiles. The third one is Negroid.
The terms are derived from the region of the world in which they were prevalent. Caucasoid, from Caucasus, is generally what we commonly call “white”.
Mongoloid, from Mongolia, is generally what we call “asian”. Negroid, literally translates as "black resemblance" from negro, is what we generally call “black”.
Of course, continued intermingling (and ultimately a common original ancestor) means that these distinctions are more arbitrary than scientific. Differences in physical characteristics are usually thought to have derived from the climatic conditions of life in each region.
See https://en.wikipedia.org/wiki/Negroid for a very good chart of the distribution of each group.