Concave is a shape the opposite of convex.
If any two points can be connected by a line that goes outside the shape, the shape is concave. The letter C is a concave shape. An easy way to remember concave is thinking of it being like a hill with a cave in it. If you put a straight line between a gold nugget in the floor and a diamond in the ceiling, the line is not inside the hill anymore (it is in the air not in the earth). The inside of the cave is concave.
The word concave means "with a cavity."