They are the new spacing utility classes. I think they’re very intuitive:
p-*
is for padding all sides.m-*
is for margin all sides.pl-*
is for padding left.mt-*
is for margin top.mr-auto
is for margin right auto.
Spacing Utils Demo
There are also display utilties.
d-block
, d-flex
, d-inline-block
, d-none
etc…