HM Revenue and Customs (HMRC) is like a giant piggy bank that the government uses to collect money from lots of different people and organisations. This can include money that you owe them, like taxes, or money you get back from them, like a tax refund.
Think of it like this: when you get pocket money from your parents or grandparents, they might keep some of it for safe keeping, and later on they might give you some of it back if you've done something good. HMRC works the same way, but on a much larger scale and with lots of different rules they have to follow.
HMRC also helps to make sure everyone is playing fair by checking that people and businesses are paying the right amount of tax, and they can take action if they think someone is cheating. They also help to pay for important services, like schools, hospitals and roads, with the money they collect.
Overall, HMRC is a bit like a big organisational piggy bank that helps the government to manage its finances and make sure everyone is doing their bit to contribute.