I don't understand what the textbook means here, or why it works. Thanks
Tbh, it's not super important - this is more just motivation as to why we care about chirality in molecules. That is, it has this control over the way light bends, and the "opposite" (in some sense of the word) chirality will cause it to bend the other way.
Essentially, I'm going to shine some light at a wall. We know that light is a wave (like a sine wave), but it can travel in lots of different directions. If you put it through a polariser, all the light disappears except for the light travelling in one specific direction, which is controlled by the polariser. We call this the polarised light.
If you take this polarised light, and put a chiral molecule in front of it, then you can do some experiments that will tell you that the light is now travelling in a different direction. That's what the books trying to get across.
What might be confusing you is when I keep saying the waves of light are travelling in different directions (with the polarised light only travelling in one direction), but we only have one beam of light - it's only travelling forwards. So even though the light is travelling forwards, the waves in the light could be oscillating in lots of different directions. It's this oscillating that we keep track of when polarisers become involved.
But as I said - it's not super important you know this stuff, it's more important that you just know about it, and give some sort of motivation as to why we care about chirality/some of the properties of chiral molecules