In English, those two words can be used interchangeably in colloquial usage, they're synonyms. This would make sense if "thật" translates to the meaning of "really" that is a synonym for "truly", but there isn't enough context.
Somewhere in this course's comments I read that plurals with "Những" are more like indefinite and with "các" are more like definite. Can someone elaborate more on this?
I think các is for all members of a group (oranges are orange) whereas những is for some members of a group (some oranges are green). I'm still pretty new at this though so someone else should confirm!