Mesopotamia in Bible times was like the ancient version of the buzzing New York City, only much bigger. It was the vibrant hub of a civilization that emerged from land sandwiched between two rivers, the Tigris and the Euphrates. That's mainly in Iraq and western Iran.
This area was prime real estate for early human settlers because those rivers brought life to the arid land. The word "Mesopotamia" itself means "between the rivers" in Greek. It's a fair name because these rivers provided water for farming and transportation.
Mesopotamia such a big deal in Bible times because it became the stage where many biblical stories played out. This was where we would have found:
the famous city of Babylon
the Tower of Babel (that's the one where people tried to build a tower to reach the heavens, and it didn't go too well).
home to Abraham (city of Ur) the father of three major religions: Judaism, Christianity, and Islam. He came hailed from the city of Ur, which was right there in Mesopotamia.
Daniel in Mesopotamian lion’s den
Also, Israelites spent decades in Mesopotamia during their exile to Babylon's land. That's where we come across the story of the prophet Daniel and the lions' den. The Babylonian King Nebuchadnezzar, who had a starring role in the Bible by destroying Jerusalem, ruled this land too.
Mesopotamia was like the biblical backdrop for many important events in the Bible. Those rivers, the Tigris and Euphrates, made it into a thriving civilization.