What does the line above a set symbol mean?

[astrozilla]

Homework Statement

Hi
I would like you please to look my attachement ,and explain to me the meaning of the line above M and N what's the meaning of this line?It seems to me that it acts as we should take the complementary collection of numbers .

Homework Equations

The Attempt at a Solution

 

[micromass]

Yes, you are probably right. The notation $\overline{N}$ just means all the elements which are not in N.