95 maxima help, important

right behind the front 3 spark plugs, there are 3 plugs(dont know what they are exactly). they are held down by 2 screws just like the spark plugs. anyways i unscrewed 1 of them and when i pulled off the grey plastic part the silver washer fell off and i cant get it because its somewhere under the engine. now i only have that little black foam/rubber piece. how important is that flat little washer? is my car safe to drive? can i buy a new one at my local auto store? help needed asap plz, as im scared to drive my car

someone help me!!!