Presales is a way for people to help others learn about things before they buy them. Imagine you're trying to buy a toy for your friend's birthday but you don't know much about toys. You go to the toy store and ask the person who works there for help. That person is doing presales!
They might ask you questions to help figure out what kind of toy your friend would like. They might show you different toys and explain what they do. They might even let you try them out so you can see how they work. All of this helps you make a good decision about which toy to buy.
Presales can happen in lots of different situations, not just at toy stores. For example, if your grown-up friend is looking to buy a new car, they might talk to someone at the car dealership to learn about different models and features. That person is also doing presales!
Basically, presales is when someone helps you understand something before you decide to buy it. It's like having a really knowledgeable friend who wants to help you make a good choice.