The Big Bang is a theory about how the universe started.
Currently, the physicists estimate the universe started 13.7 billion years ago.
The astonomers observing the stars found out in 1912 that some nebulae and constellations were moving away from the Earth. Something must have caused them to be moving at such speed. Then they decided there must have been a huge expansion point in the Universe at some time. They called that the Big Bang