They are print bricks, you can find a print brick like the 1x1 print brick in the 'Bricks' tab at the bottom, take the print brick, place it, get out your 'Tools' (Default key is Q), then go to your 'Printer' item, use it on the print brick and from the menu that pops up, you can choose a letter or some other print, keep placing 1x1 Print Bricks and using the Printer on them until you've spelled out a word or whatever you're trying to do