Minecraft wiki entry for Development_versions, associated text: A technical block.
Not obtainable in survival.
Is a junction block that allows to construct structures out of smaller templates.
Target pool refers to a pool of elements the jigsaw block can draw elements from (ie: the next template to place).
Attachment type refers to the "name" of the jigsaw block. Jigsaw blocks can only connect to other jigsaw blocks with the same "name" (attachment type).
Turn into is the blockstate the jigsaw block will turn into once the whole feature is placed.