take 123.456

the 1 is in the hundreds position, so it is expressed as 1 x 10^2

for the 2, it is 2 x 10^1

the 3, it is 3 x 10^0

the 4, it is 4 x 10^{-1}

the 5 is 5 x 10^{-2}

the 6 is 6 x 10^{-3}

that's all you teacher was saying and obviously this is true. if you sum all the things i gave you together you get 123.456