I think they're talking about three-dimensional art in which a piece or pieces are cut out of a geometric shape and rearranged, as in the first picture here:

http://www.angelomuriotto.com/it/neodinamismo/

Here's an even simpler example from the same artist:
http://www.angelomuriotto.com/wp-content/uploads/2019/07/L3A...

The "forma matrice" is the original shape - I would be inclined to leave out "form", as it doesn't add much, but it's up to you.

I think both of these definitions fit the context:

Matrix
1: something within or from which something else originates, develops, or takes form
an atmosphere of understanding and friendliness that is the matrix of peace
2a: a mold from which a relief (see RELIEF entry 1 sense 6) surface (such as a piece of type) is made
http://www.merriam-webster.com/dictionary/matrix