Numpad 0 in default config I think. It's from the WAILA (What Am I Looking At?) mod.
yup, it's from WAILA. it used to be a mod that integrated with NEI's tooltips, but now has moved forward to being independent from NEI.
by pressing "0" you're able to change certain configurations of the mod, for example you're able to move the tooltip, make it more opaque or changing the size of the tooltip.
you can disable it with "1", you can turn on/off liquid tooltips with "2" and you can view the recipe of a block with "3" and recipes where the block is used in with "4"