:help i_CTRL-D
:help i_0_CTRL-D
:help i_CTRL-T
Remove indent level.
In this example you can see how the indent level gets reduced. The great thing is that it doesn't matter where your cursor is in the line.
If you write a 0 (zero) and then press <C-d> then all indent levels are removed (and the zero). Weird, right?
