Confused with Navbar-fixed-top

HTML-CSS
1

Hi, please help me figure out why navbar-fixed-top swallowed the top part of my main conten, which is the first heading (

Brief description of my programming repertoire

).

Here’s my code:

Thank you in advance!

3

Thank you! One more question… what does it mean Make sure to include this after the core Bootstrap CSS.? What constitutes the core Bootstrap CSS?