html - What does this css declaration mean - two declarations at once -

Can you please explain this kind of CSS declaration:

  Menuu_blueSkin_Middle.dir_left div .align_left {float: left; }   
 As far as I know that you have  .someName {} , this means that this HTML element has been put in the class attribute. For example:  
  & lt; Div class = 'someName' & gt; & Lt; / Div & gt;   
 But in the example that answers this question, there are two points in the beginning and in the middle again a space and another declaration is div.align_left?  
 Please give me some clarification!   
 
 
 . Menuu_blueSkin_Middle.dir_left div.align_left   
 There are three squares called here.  
 This is a bit complicated because whoever nominates these sections is an amateur.  
 To make it easier to understand, rename the three classes:  
 
  menu_blueSkin_Middle  we .  
  dir_left  We will rename  .secondclass  for the first class  
  
  .lign_left  for  .thirdclass       
 
 The name will change, well, now the class named after all three, shows you the code:  
  .firstclass.secondclass div.thirdclass {float: left; }   
 
 OK so the following applies:  
 
. Third class is only one [with float: left; Code].  
 Only the third class squares will be affected within a div. (Due to the location being located before)  
 The only .thirdclass div is located within a class with a double declaration of both .secondclass and .firstclass will be affected.   
 Example Code:  
  & lt; Div class = "firstclass secondclass" & gt; & Lt; P class = "third class" & gt; & Lt; / P & gt; & Lt ;! - Note: This is only a note, so you know that is affected. The division below ** is only an impression **. The top p is not affected because the DIV is not affected in the lower part because it is not a div. Because it is not nested in the first class second class - & gt; & Lt; Div class = "third class" & gt; & Lt; / Div & gt; & Lt; / Div & gt; & Lt; Div class = "third class" & gt; & Lt; / Div & gt;