Why this happen the div's margin inside a div is overflow i read about it and i guess it margin collapse something like that ...How do i fix it??

How is anyone supposed to know "why" that is happening without seeing your CSS? I don't think anybody is going to be able to know exactly what you've typed so far and then tell you why it isn't working just from looking at that picture.

It happens because margins are outside of your element nested in the topbar. This topbar has a fixed height that is smaller than the height of the ul-element along with its top and bottom margins.

There are tons of ways to fix this, bit I am not sure what would be your desired outcome, along with the ul behavior

Oh and welcome aboard