The Israelites were in Egypt because a famine brought them there. God used the many bad things that happened in Joseph’s life to provide for them by putting him in leadership in Egypt. (You can read through Genesis 37 for the background story of how they got to Egypt.)
Over time, different Egyptian Pharoahs decided to use the Israelites as slaves. They lived that way for over 400 years.
Then, God raised up Moses to rescue them. God used many miracles to help convince Ramses, the Pharoah of that time, to let His people go. Once Ramses agreed, he immediately regretted his decision.
There were over a million Israelites. That’s like the city of Atlanta all leaving and walking together down here to my town in south Georgia.
They got to the Red Sea and thought it was over. At the widest point, the Red Sea is 200 miles wide. Ramses and his men were right behind them in their chariots.
How were a million people supposed to get across that? God sent a wind and pushed back both sides of the sea in order for them to walk across, on dry land. Imagine being in an aquarium in a tunnel, like this: