Nothing, but English requires an article.
You can use indefinite or definite, High Valyrian distinguishes these only by context.
I suppose when people say mother you assume they are referring to their mother, so you need the article to distinct that it's not.
http://mylanguages.org/finnish_articles.php The Finnish language for example also lacks definite and indefinite articles. The only way you could define objects would be using separate words like "that" and "this". For some one who spoke a language with no articles, it did take some time to get used to using them