What is this light grey LEGO plate with six sides (hexagon)?
Informally, it's known as the Nexogon.



Bricklink calls it "Plate, Modified 6 x 6 Hexagonal with Pin Hole". The part number is 27255. It's original use was as a shield in the Nexo Knights line in 2017, but it has appeared in a few other sets since then.



If you're interested in ways to use this part, New Elementary did a whole series of posts from different builders when it was first released. The unique shape and different connection families open up a lot of fun opportunities.