的 de is the most common word in Chinese. It is frequently
used as a grammatical marker to link a noun and its modifier.
For example, in 这是我的名片 (Zhè shì wǒ de míngpiàn), This is my name card,
the grammatical marker 的 links the noun 名片 with the modifier 我 indicating possession, i.e. my name card.
Similarly in 我的母语 (wǒ de mǔyǔ) my mother tongue, the noun 母语 is modified by 我 indicating whose language the phrase is about.