A grid is usually a structure made up of vertical and horizontal lines that intersect, used to structure content.
For example, this page uses a grid structure that I've changed to fit my desires. Grids are used in various software to organize data or information.
Being responsive means that the page will conform to the hardware that a user is using. This ensures that no matter the hardware (phone, tablet, pc or mac) the website will still be readable. If you wish, try to change the width of the page. This page should conform properly.
Semantics in HTML refers to the relationship between elements in the HTML code. These relationships allow us to structure the website through organizing elements, each with its own description and function.
This has given me some insight on presentation. While I've used the bootstrap CSS file, I hope to use the info to create my own CSS files.