1.
=$B6
dragged down,we get successively the contents of
B6
B7
B8
.
.
.
This is the intent of the set up: the column component is fixed while the row component increments.
Dragging to the right, this will return the contents of B6 repeatedly in each successive cell.
2.
=B$6
dragged to the right, we get successively the contents of
B6, C6, D6, E6, ...
This is the intent of the set up: the column component increments while the row component is fixed.
Dragging down will return the contents of B6 repeatedly in each successive cell.
3.
=B6
dragging to the right and the down will yield:
B6, C6, D6...
B7, C7, D7...
B8, C8, D8...
4.
=$B$6
gives just the contents of B6 when dragged down or when dragged to the right.
Hope this helps.