How it works
Each channel is a whole number from 0 (none) to 255 (full). rgb(255, 0, 0) is pure red, rgb(0, 0, 0) is black and rgb(255, 255, 255) is white. Mixing the lights adds brightness, so equal amounts of the three give greys.
Relation to HEX and HSL
A HEX code is the same three numbers written in base 16: rgb(59, 130, 246) is #3B82F6. HSL describes the same colour by hue, saturation and lightness. The Color Converter moves between all three.
Common misunderstanding
RGB is additive light, which is why mixing red, green and blue paint does not give white. Printers use a different model, CMYK, based on ink. See how to convert HEX to RGB and HSL for worked examples.