This example is the best I've come across to understand the blockchain, and I'll simplify some of it as a superficial way to explain what Bitcoin is.
If you and I meet in the park and I give you an apple then there is no doubt what happened. I no longer have an apple. You have an apple. We can both see it. My hands are empty. yours are not.
It's now your apple and we did not need another person to move the apple.
This is a little worse on the internet.
If I send you a digital apple. How do you know that I did not make more copies of the apple and sent it to others too?
You can not.
Potentially, I may have used the same digital apple many times.
Therefore, we create an accounting book.
It says that I have transferred a digital apple to you. However, I can still change the accounts and send you digital apples I do not have.
To avoid this issue, we share the accounting book with everyone else to make it public. Everyone gets the copy of our apple transaction added to the accounting book with all other apple transactions.
Because everyone has all transactions it is very difficult to change it.
In other words, I can not send apples I do not have. Because then everyone has to verify that I had that apple. But, as I gave away the apple to you, it disappeared from my hand.
Apples do not occur randomly. The total amount of apples is predetermined from the beginning.
In order to secure other people's apple transactions through the public accounting book, you can update it regularly. For that work, you can receive extra apples, but you only get it if you beat everyone else in a relatively meaningless competition.
Apples in this case are Bitcoin.
The accounting book is blockchain.
And the work they do to update the accounting book, is mining.
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@Magnulf