RGB is an additive colour space, with coloured red, green and blue lights added to a black screen to produce the colour you see. With all 3 colours combined at their maximum value, the result is white.
Many design tools will often default to an RGB workspace when creating a new document. RGB is great for creating digital graphics – the colours are bright and vibrant when viewed on-screen but may not convert accurately when printed.
If you are creating graphics for web, email, or any video media, use RGB for faithful colour reproduction.