When you have a postpaid mobile phone, it means that you have a phone plan where you pay for your phone service after you use it each month. Imagine that you have a toy box full of little toys, and each day you decide to play with some of them. At the end of the day, your mom or dad checks which toys you played with and then asks you to pay for those toys that you used.
Similarly, with a postpaid phone plan, you use your phone to make calls, send texts, and use data (like playing games or using apps). At the end of the month, your phone company checks how much you used your phone and then sends you a bill. This bill includes how much you owe for your phone service (your calling, messaging, and data).
Postpaid phone plans also usually have a contract (a little like a promise) between you and the phone company. The contract might say how long you have to use the plan (like a year), and it might say what you need to pay if you decide to stop using the phone plan early. The contract also sets up rules for what you can and can't do with the phone service (like using your phone overseas or transferring your phone number to a new provider).
So, in short, a postpaid mobile phone is like having a toy box with a parent who asks you to pay for the toys you use at the end of each day. But it's your phone service, and you make a promise to the phone company to pay for it.