Standard-dynamic-range video is a representation of light intensity based on the cathode-ray tube's brightness, contrast, color characteristics, and limits. It's able to represent a video or picture's colors with a maximum luminance of around 100 cd/m2, a black level of around 0.1 cd/m2, and Rec.709 / sRGB color gamut. It uses the gamma curve as a transfer function.
SDR videos and images are supposed to be watched on a display that has the same characteristics and limit as CRT. However, it's technically possible to show them with higher brightness levels, deeper blacks, and more saturated colors by adjusting the SDR image. This can however lead to quality issues, the content creator's intents can't be ensured and the display becomes out of calibration.
HDR is a new way of representing the light that emerged in 2014 to overcome SDR's limits.
The first CRT television sets were manufactured in 1934 and the first color CRT television sets were manufactured in 1954.
While conventional gamma curves are useful for low light and are compatible with CRT displays, they can only represent a limited dynamic range. Standards require SDR to be viewed on a display with the same characteristics as CRT (i.e. 100 nits peak brightness, gamma curve, Rec. 709 color primaries). However, current displays are often way more capable than CRT's limits. On such display, higher brightness and wider color gamut can be displayed by adjusting and trying to enhance the SDR picture. HDR is however required for the creative intents to be preserved.