For non-Koreans. Background info. and things you probably missed.
Mrs. Kim:
Since Mrs. Kim had been playing a role of a mother in T.V series called 'farm dairies' for more than two decades, she has become an ultimate icon of 'the mother' in South Korea. In the show, she was shown as a traditional, thoughtful, and wise mother who deeply loves her sons.
In the movie, she still plays the similar kind of mother but you know there's twist. Such twist greatly thickens the plot and one of the fundamental questions the film asks: the mother, as usual, gives unconditional love toward her offspring but can such love justify the injustice?
For non-Koreans wouldn't know what Mrs. Kim represents(it's not even something Koreans know or understand, her image of being the mother hits them from the unconscious level, the film loses great portion of its sense. This is something you should keep it mind.
................................................
The significance of ending:
'Thank You Parents Bus Tour' is where bunch of parents from country-side go on a few days of tour across the country. As you see in the film, they sing and dance crazy in the bus. It's like being in a club except it's actually a freaking bus. It's a common cultural practice among elders in country side.
The significance is that, through the brilliant camerawork and lighting, we lose track of the mother. She gradually fades away into the crowd. It's not regular crowd but other mothers. She becomes representation of other mothers while other mothers become representation of the mother. Think semiotics.
Such notion is significant when you think about what a mother and parent in general represents in South Korea. Above mentioned unconditional love is seen as the greatest thing ever. However, it has its own dark side. Generally speaking, parents are blindly in love with their children that they produce very spoiled kids.
You may ask, regardless of where you came from, it's the same issue in all across the globe but South Korea really stands above on that notion. If you have lived in South Korea, you'd get a better sense of this issue (I've heard China is pretty much the same.)
--------------------
The director:
One thing you should keep it mind is that the director, Joon-ho Bong, always injects very strong yet subtle social commentary notions in his films. Those who do not know history, culture, issues of South Korea would miss great deal of what lies underneath.