What Do You Mean Mass is Energy?

As a kid I had no idea what it meant when I was told "Mass is Energy" and "Mass can be converted into energy". No matter how many times someone repeated that "They are just the same thing" it didn't click.

Well hopefully this can clear things up to younger me.

Further Reading:

Inside the Proton, the ‘Most Complicated Thing You Could Possibly Imagine’ - https://www.quantamagazine.org/inside...

Mass the Confined Movement of Energy - https://doi.org/10.4236/jmp.2017.86058

Visualizations of Quantum Chromodynamics - http://www.physics.adelaide.edu.au/th...

Visualizing the Proton: Animation -    • Visualizing the Proton: Animation