A decision problem is a question that has a yes or no answer, but it's not always easy to find out the answer. It's like when your mom asks you if you want to eat vegetables for dinner – the answer is either yes or no, but you might have to think about it before you decide.
In math and computer science, decision problems are questions that can be answered by a computer program, but the program might have to think for a long time before it can give you the answer. Some decision problems are really hard to solve, like trying to figure out if a really, really big number is prime (can only be divided by itself and one) or not.
Scientists use decision problems to study how computers can solve different kinds of problems. They want to know how fast computers can solve different decision problems and if there are any problems that computers just can't solve no matter how hard they try. Decision problems are important because they help us understand what computers can and can't do, and they also help us create better computer programs in the future.