The backspace option determines the behavior of pressing the backspace key
(<BS>). By default, Vim's backspace option is set to an empty list.
There are three values that can be added that each independently alter the
behavior of the backspace key. These are indent, eol, and start.
When indent is included, you can backspace over indentation from
autoindent. Without it, Vim will not allow you to backspace over
indentation.
When eol is included, you can backspace over an end of line (eol)
character. If the cursor is at the first position of a line and you hit
backspace, it will essentially be joined with the line above it. Without
eol, this won't happen.
When start is included, you can backspace past the position where you
started Insert mode. Without start, you can enter Insert mode, type a bit,
and then when backspacing, only delete back as far as the start of Insert
mode.
The backspace default is absurd, you are going to want to add all of the
above to your Vim settings.
See :h 'backspace' for more details.