Sounds like you might be new to the memory palace technique. I’d recommend using the search function for some really helpful content about them and also check out the FAQ section.
Basically, a memory palace can be a mental representation of a location (real, fictitious, or completely imagined) that you picture traversing via a consistent and logical path. Along the way, you can pick out sub-locations which could be something like rooms within a house, or general areas or zones which contain individual points of interest (commonly referred to as loci.)
So for example, my house is the palace. The living room is one room or sub-area. The bathroom is another. The kitchen is another. The couch is a loci within the livingroom sub-area, within the house palace. The stove is a loci within the kitchen. The toilet is a loci within the bathroom.
By pre-planning the content to memorize, you’ll know how many loci or areas you’ll need to plan in your palace. If you designate a room for each chapter, you may need a larger palace with more areas, or multiple palaces connected in a logical way to get enough areas to work with.
Once you have your loci and pathway picked out, you can “place” images (by visualizing activity at the loci) that represent the content you want to memorize.
For example if the 10th chapter’s main points are the nautilus being attacked by a giant squid and captain nemo playing the organ (it’s been a looooong time since I’ve read it, so forgive the inaccuracies) you’d imagine your tenth area, say the stove in your kitchen, and on that stove you might imagine a boiling pot with a squid swimming inside, attacking you every time you go near it. All the while, organ music is playing and when you peek inside the oven you see a ship captain playing the organ.
This is what its meant by placing images.
Now when you go to review and recall the content, you’ll imagine getting to the stove, its your 10th area so its chapter 10, and hopefully the scene thats there is vivid enough that you’ll easily remember squid attack and organ playing.