Divergent

I am reading a book called Divergent.There are 487 pages in this book.  The author of this book is Veronica Roth. This is the first book in the Divergent series. Divergent is about a girl named Beatrice who is from a faction called Abnegation and it is almost time fore her choosing day. Before the choosing day she takes the aptitude test to see which faction she belongs in. When she takes the test her results were unclear meaning she is a divergent. On her choosing day she chooses to be a Dauntless and changes her name to Tris. When she is in Dauntless she meets new people. When she goes to the Dauntless compound she needs to got through training to see of she is allowed to stay in Dauntless or else she will become Faction less. 

Two words I didn't understand;

Skim 
Skim means go or move quickly and lightly over or on a surface or through the air.

Undercurrent
Undercurrent means an underlying feeling or influence, especially one that is contrary to the prevailing atmosphere and is not expressed openly.