Teachers notes layout tips

Hi, I've managed to complete the challenge however can't really understand what the tips in the teachers notes below do, even Guill didnt use any of these in his method.

Column layout tips

1)To add a left gutter to all but the first column, you can use this:

.col + .col { padding-left: 1em; }

(I understand this adds 1em of padding to the left but why target .col twice?)

2)To remove the right gutter from the column closest to the right edge of the page, use :last-child

.col:last-child { padding-right: 0; }

(What does last-child do? does this suppose to clear any padding/margin at the right edge of the page ?)

(3)Bringing it all together This snippet floats all columns left and applies a right gutter. It applies a left gutter to all but the first column, so the first column will be flush with the left margin of the page. Then it removes the right gutter from last column, so that it's flush with the right margin of the page.

.col { float: left; padding-right: 1em; }

.col + .col { padding-left: 1em; }

.col:last-child { padding-right: 0; } (What does flush exactly mean?) Sorry for making a lot of questions but I wish to completely understand before I move on.