Looking at the photos I think you can guess what the object of the puzzle is; yes, to get the cubes to stack atop one another and the smaller block to fit inside the wenge cube. The cubes have been constructed in such a way that there is a particular order for stacking them; maple at the bottom and wenge on top with the cherry in between. Any other combination and the stack will have gaps.
When I first looked at the puzzle, I thought it was going to be a really tough challenge given there are 4 pieces. And in the early stages I had some difficulty with all the cubes rolling around inside the jar. But as I puzzled on, turning the jar in all directions, it didn't appear as difficult as I had expected. I slowly got the hang of it. I could get the cherry onto the maple cube and with some persistence, the wenge thereafter. The part that was most difficult was to get the small block into the cavity of the wenge cube. This took me a good twenty minutes to get right and occasionally in between, the other 3 cubes would dislodge and I had to start all over again.
Trios Chocolats is a rather interesting design from Frederic and as i can testify, not impossible to solve; in fact surprisingly manageable, so long as you keep at it. For those into dexterity or bottle puzzles, here's one worth collecting.