css multiple selectors explanation

I m trying to get my head around CSS.
I found a navigation menu that I would like to use but some things aren't working yet and there are a lot of things that confuse me.

is there a reason why li is two times in this part of the code ?

/*Make dropdown links vertical*/
li ul li {
display: block;
float: none;
}

and this part I don't understand at all.
does this mean hover state for only a selector or does it mean hover state for ul, li and a selectors ?

ul li a:hover + .hidden, .hidden:hover {
display: block;
}

I have been following the css tutorial from w3schools but I didn't see anything that explains the things I don't understand.
Full code is here

/*horizontal navigation style*/

/*Strip the ul of padding and list styling*/
ul {
list-style-type:none;
margin:0;
padding:0;
position: absolute;
}
/*Create a horizontal list with spacing*/
li {
display:inline-block;

float: left;
margin-right: 1px;
}
/*Style for menu links*/
li a {
display:block;
min-width:140px;
height: 50px;
text-align: center;
line-height: 50px;

font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
color: #fff;
background: #2f3036;
text-decoration: none;
}
/*Hover state for top level links*/
li:hover a {
background: #19c589;
}
/*Style for dropdown links*/

li:hover ul a {
background: #f3f3f3;
color: #2f3036;
height: 40px;
line-height: 40px;
}
/*Hover state for dropdown links*/
li:hover ul a:hover {
background: #19c589;
color: #fff;

}
/*Hide dropdown links until they are needed*/
li ul {
display: none;
}
/*Make dropdown links vertical*/
li ul li {
display: block;
float: none;
}

/*Prevent text wrapping*/
li ul li a {
width: auto;
min-width: 100px;
padding: 0 20px;
}
/*Display the dropdown on hover*/
ul li a:hover + .hidden, .hidden:hover {
display: block;
}

/*Style 'show menu' label button and hide it by default*/
.show-menu {
font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
text-decoration: none;
color: #fff;
background: #19c589;
text-align: center;
padding: 10px 0;
display: none;
}

/*Hide checkbox*/
input[type=checkbox]{
display: none;
}
/*Show menu when invisible checkbox is checked*/
input[type=checkbox]:checked ~ #menu{
display: block;
}
/*Responsive Styles*/
@media screen and (max-width : 760px){

    /*Make dropdown links appear inline*/
    ul {
    position: static;
    display: none;
    }
    /*Create vertical spacing*/
    li {
    margin-bottom: 1px;
    }
    /*Make all menu links full width*/

    ul li, li a {
    width: 100%;
    }
    /*Display 'show menu' link*/
    .show-menu {
    display:block;
    }
}

Answer

li ul li {
  display: block;
  float: none;
}

It means it will target all the

elements which are inside li ul..The css are always applied to the last selector in the expression. See example below

Stack Snippet

li {
  color: blue;
}

li ul li {
  color: red;
}

  
Menu
    

      
Submenu
      
Submenu
      
Submenu
    
  

---

ul li a:hover + .hidden, 
.hidden:hover {
  display: block;
}

Here you are applying same styling to two elements using ,(comma) separator...

1: using + i.e adjacent sibling selector...it separates two selectors and matches the second element only if it immediately follows the first element, means here and .hidden sholud be both adjacent elements.

2: is .hidden:hover means it will work only when you hover only .hidden element.

.hidden {
  display: none;

}

ul li a:hover+.hidden {
  display: inline-block;
  background: red;
}

  
Hover Herehidden