Alright kiddo, have you ever asked your mom for something she said no to, but then you asked for something smaller and she said yes? That's kind of like the foot-in-the-door technique!
It's a way to get people to say "yes" to bigger requests by first asking for something smaller. This works because once someone has said "yes" to a request, they are more likely to say "yes" to another request that the same person asks them for.
Let's say a salesman comes to our house and asks if we would answer a few questions for a survey. We say "yes" and answer his questions. Then, he asks if we could listen to him talk about his product for a few minutes. Since we already said "yes" to the survey, we might be more likely to say "yes" to the product pitch too!
So, the foot-in-the-door technique is all about starting with a small request, getting a "yes" response, and then asking for a bigger request which is more likely to be granted.