The empty triangle — bad shape
Go has classic 'shapes' — arrangements of a few stones that are more or less efficient. The most famous BAD shape is the empty triangle: three stones in an L that enclose no useful empty point, wasting one stone's worth of efficiency and often running short of liberties. Good shapes (the table shape, the tiger's mouth, the wall) use the same stones for more effect. Learning a handful of shapes makes your moves 'look right' almost automatically.