1. courage or resolve
a display of grit
As a verb to have 'grit' is to have a lot of determination or dedication to something especially when results do not come early on. You need grit to succeed many things, grit is also referred to colloquially as 'stick with it-ness'.
2. small loose particles of stone or sand
grit in your eye
As a noun, 'grit' refers to small particles of sand or stone. You need to grit the roads if there is ice or snow for example by laying down salt or sand.