Boggart takes the shape of the thing the viewer is afraid of.
Boggarts lurk in cupboards and when confronted, take the form of your worst fears.
In Fantastic Beasts: The Crimes of Grindelwald, when Leta Lestrange faces the boggart, it takes shape of some white cloth floating. That scene moves and it doesn't get cleared what is it and why she fears it.
So, is there any explanation what exactly that thing was and why she fears it?