The Communion of Saints is family. Seriously, they’re a bunch of older and wiser brothers and sisters. They lived well, God has made them perfect, and they want nothing more than to give glory to God and help out their journeying little brothers and sisters. They’ll pray to God for you, if only so that God can be more glorified through you.
Even in this life we ask people to pray for us. The saints, by the grace of God, are able to do the same.
Mary is a saint, but in many ways she is like an adoptive (human) mother, and that’s in two respects. (1) We are raised again as brothers and sisters of Christ, who is her son, and (2) she’s like the Queen Mother of Israel, mother of the king and an advocate of the people. She’s not God. She just wants to pray to God on our behalf.
Consider the words of the Hail Mary: “Hail Mary, full of grace, the Lord is with you. Blessed are you among women, and blessed is the fruit of your womb, Jesus.”
That is basically verbatim quotations from Scripture, the words of the angel at the annunciation and the words of Elizabeth when Mary visited her. As Mary declares in the Magnificat, all nations will call her blessed because of Christ.
The second part of the prayer,
“Holy Mary, Mother of God, pray for us sinners now and at the hour of our death.” This is simply us asking for her to pray for us, like we do with the saints, and like we do with pther Christians in our daily lives.