Learn Arabic (Modern Standard): يا أمي، انتظري هنا.

Because the speaker is giving a command to a woman (their mother). When you tell a single female to do something in Arabic, you add the suffix ـي to the end of the base command form. If you were telling a man to wait, you would drop that ending and just say انتظر.

Why does the command انتظري start with an alif (ا)?

Many command verbs in Arabic start with this helper alif. The core of this verb starts with two consonants, which is hard to pronounce at the beginning of a word in Arabic. The alif is just there to provide a starting vowel sound so you can say the word smoothly.

Why do we use the word يا before أمي?

In Arabic, whenever you speak directly to someone to get their attention, you use the calling word يا before their name or title. English doesn't really do this anymore (it is like saying "O mother"), but in Arabic it is completely standard and polite.

How does the word أمي mean "my mother" instead of just "mother"?

The base word for "mother" is أم. To make it "my mother," you attach the possessive suffix ـي to the end. Notice that this is the exact same letter we added to انتظري to make the command feminine, but it does two completely different jobs depending on whether it is attached to a noun or a verb!