Explain the dramatic irony of lines 100-112 why is macbeth's action significant

First of all, you need to know what dramatic irony is. Dramatic irony is a technique where the audience and some of the characters know something that the other characters do not.

In this particular scene, we see Macduff who enters king Duncan's room, only to find that he has been murdered. Macbeth, Lady Macbeth, and the audience obviously already know who killed him, but are pretending not to know so as to maintain the appearance of innocence. Macduff doesn't know who killed the King, which is why this is dramatic irony. It is important for Macbeth to pretend he is horrified so that nobody suspects that he was the one who killed Duncan.