Shmi is Anakin's mommy, and one of Watto's slaves. She represents the prototypical kind mother character. She loves her son and raises him to be a caring, giving person. She probably bakes a mean Tatooine-style apple pie, too.
We can tell that she's a maternal goddess when we first meet her. Anakin brings home a Jedi, Jar Jar, and a queen disguised as a handmaiden, and she welcomes them into her home for dinner. (Yes, even annoying Jar Jar.)
Shmi further demonstrates her willingness to put the needs of others before her own when she lets Anakin race to win Qui-Gon the money to repair his ship. While she hates watching her son race—who wouldn't? it's crazy dangerous—she relents, saying:
"There is no other way. I may not like it, but he can help you. He was meant to help you."
Shmi's final and most daunting act of kindness comes after Anakin wins his freedom. Obviously she doesn't want to lose her son, but she's willing to let him go so that Anakin can have a better life far away from Tatooine. In this way she contrasts with Uncle Owen from A New Hope, who thought he could keep Luke safe by keeping him on Tatooine.
Anakin is also worried about losing his mother, but Shmi tells him:
"But you can't stop the change any more than you can stop the suns from setting."
It's her final lesson to Anakin: change brings unavoidable loss that you have to accept in order to live a good, happy life. It's a lesson she lives by example at that moment.
Unfortunately, as we'll see in the later prequels, Anakin doesn't take the lesson to heart and his struggle to halt change, and therefore loss… directly leads to him becoming Darth Vader.
Shmi Full of Grace
In terms of the saga's mythology, Shmi plays a minor but important role in the Star Wars universe. Once Qui-Gon realizes Anakin is Force sensitive, he asks Shmi about the boy's father. She responds,
"There was no father. I carried him, I gave birth, I raised him. I can't explain what happened."
This draws parallels to the story of the Virgin Mary. In Catholic theology, Mary being a virgin when she gave birth to Jesus is important because it meant that her son was free of original sin. This is referred to as the Immaculate Conception.
Shmi being the Virgin Mary of the Star Wars universe signifies the importance of Anakin's birth. For Qui-Gon, it is proof that Anakin is the one foretold in prophecy that will bring balance to the Force.