The definition from Merriam-Webster's dictionary describes bitcoin as a cryptocurrency and payment system invented by an unidentified programmer, or programmers, plural, under the name of Satoshi Nakamoto. It was first introduced in 2008 through a mailing list, then later released as an open-source software in 2009. To the average person, this doesn't explain much.
Imagine a real life scenario in which you are holding one apple.
If someone standing next to you asked for this apple, you would physically place it into their hand. The apple would then be in their hand, you both would know that the apple is in their hand, and it now clearly belongs to them. Your apple is gone and you're no longer able to give anyone else another apple, simply because you have none left. As with any other object in an in-person exchange, this is a very clear cut situation.
Now imagine a scenario in which you have a digital apple on your computer.
You can email someone the apple, and now they have it too. But it’s obviously more complicated than that because no one knows if you still have the original digital apple or if you've also sent copies of that digital apple to anyone else in this room.
This problem is what is called the double-spending problem that often occurs in digital transactions, and bitcoins offered a solution.