Qiyas is a big word that means "analogical reasoning" in Islam. It is like when you try to find an answer to a question by comparing two things that are similar to each other.
Let's say your mom doesn't let you eat ice cream before dinner because it will ruin your appetite. But, you really want to have some. You remember that last time you ate a big bag of chips before dinner, you didn't feel like eating anything else. So, you use Qiyas to compare eating chips to eating ice cream. You tell your mom that just like chips, eating ice cream will make you lose your appetite, and you won't be able to eat dinner. So, your mom lets you have a small cup of ice cream, because your Qiyas made sense to her.
Qiyas is often used in Islam to help understand new things that aren't specifically mentioned in the Quran or Hadith. For example, if we know that alcohol is prohibited in Islam because it affects our judgement, we might use Qiyas to conclude that smoking weed is also prohibited, because it has a similar effect on our brain.
Qiyas is not easy, and it involves a lot of thinking and comparing things to find the right answer. But, it helps us make good decisions and understand the teachings of our religion better.