Primitives are default blocks and are located above the "Make a block" button, as opposed to a custom block which is below that button.

Editing primitives is only available in the developer version Snap! 10.0. In that version, open the settings menu and enable "Blocks all the way". Then right-click on a primitive block you want to edit. You should see the options to edit as if it were a custom block.

When you see the definition of a primitive, you see they all have the primitive () block. This block, keeps the primitive working, and there's a Boolean switch. If switched off, the blocks under it start working and as usual if on, it prevents these blocks from working. The scripts below that block in definitions are usually workarounds.