As long as any term can fulfil the function of a trademark, it can be registered. This rule applies to common words as well.
Take, for example, apple. Apple is a generic word, and anyone can use it. So, how come Apple Inc. managed to trademark it?
Since the word apple does not relate to electronics or computers, it can be considered arbitrary in connection to such products. The word apple does not describe any products or services sold by Apple Inc., and that's why it was easily trademarked. It is considered unique, and thus its registration was allowed.
However, if an apple seller wished to trademark the word apple, it would not be allowed since that would prevent other produce sellers from using a word that is very common within their niche - an unfair advantage for anyone to have. In this case, apple would have a descriptive character, and registration would be denied.