Albert Einstein's theory of relativity completely changed how we think about space and time. Here are some important ideas that had a big impact on modern physics:

1. The Speed of Light is Always the Same: One of Einstein's biggest ideas is that light always travels at the same speed—about 299,792 kilometers per second. This means no matter how fast you're moving, you will always measure the speed of light the same way. This discovery led to some surprising ideas about time and distance.

2. Time Dilation: Because the speed of light stays constant, time doesn’t tick the same way for everyone. For instance, if you could travel really close to the speed of light, time would slow down for you compared to someone who is on Earth. This means you could come back from your trip and be younger than your friends!

3. Mass and Energy are the Same: The famous equation $E = mc^2$ shows that mass (how much matter something has) and energy (the ability to do work) are two sides of the same coin. This idea helped us understand nuclear energy better and has influenced many areas in science.

These concepts have changed fields like astrophysics and how we view the universe, making Einstein a major figure in the history of science.