H grade stands for 'Hard' and they are good for shading. You can't press on hard pencils or you will ruin your paper. Higher numbers on H pencils means the graphite gets harder and lighter. Personally, I don't use my H pencils very much. B stands for 'Black'. A 2B is a very user friendly pencil when you are learning to draw. It can give light or dark strokes, depending on the amount of pressure you use. Higher numbers on B pencils means the graphite gets softer and darker.
HB is in between a 'H' and a 'B'..

It's the term used to describe the quick strokes of a pencil to capture movement. You won't see much detail because they are done rapidly.

There are a couple of ways you can do a little test, to see where you are going wrong. Turn it upside down which gives you a new angle and perspective to view. Look at it in the mirror where errors will easily stand out. Use reference material that is already black and white, this helps you to see the proper values. Wait one day to view the drawing from a completely new point of view. This is my favorite go-to for finding mistakes.