Chandler is like a big notebook that helps people keep track of all their important things to do and information. It's called a "PIM" which stands for "Personal Information Manager," which is just a fancy way of saying that it helps people manage their personal stuff.
So let's say you have a lot of things going on in your life - you have homework to do, a soccer practice on Wednesday, and a birthday party to go to on Saturday. Chandler helps you remember all these things by letting you write them down in different places. You can keep a list of all the homework you have to do, put the soccer practice in your calendar, and write the birthday party information in a special spot just for party details.
Chandler also lets you check things off as you complete them (like crossing them off a list). This can help you feel really proud of yourself when you finish something important.
Overall, Chandler helps people keep track of all the things they need to remember to do and keeps everything organized in one place.