Magnification answers the honest question: how many times bigger is the image than the thing itself? As a ratio of heights, $m = \dfrac{h_i}{h_o}$ — read aloud: m equals h-i over h-o. The subscripts are just the words: $h_i$ is the image height and $h_o$ the object height, both in centimetres — the same i-for-image, o-for-object convention $d_i$ and $d_o$ used last lesson. A ratio of two heights wears no units: $m = 3$ means three times taller, $m = 0.5$ means half size. The same number also hides in the distances from last lesson: $m = -\dfrac{d_i}{d_o}$ — m equals minus d-i over d-o. Two roads, one answer.

About that minus sign — it is a message, not a mistake. When a lens throws a real image onto a screen, the image comes out upside down, and a negative $m$ is simply how the math reports it. $m = -2$ reads: twice as tall, inverted. So keep the sign when the distances hand it to you, and never be alarmed by it — the picture in a camera lands upside down too, and the camera simply flips it back for you.