What does ISO 400 speed mean?

'ISO' refers to the speed at which electronic image sensors (in digital cameras) and film (in film cameras) react to light to form a picture. So ISO 400 is just one particular setting from the range of options on your camera.

'ISO' is not an acronym for a technical detail of the sensor. It simply stands for the organisation who devised the system and determined the measurements: International Organization for Standardization. The system is more or less standardized across film and digital Photography., with the same aperture and shutter speed values, a film and a digital camera set to ISO 400 would produce similar results in terms of exposure.

It's a decent rule of thumb to stick to ISO 100 (or the lowest option on your particular camera) whenever possible. The higher the ISO, the lower the quality of the image. So with a faster sensor/film (which makes low light digital photography much easier) you sacrifice some quality.

Whilst digital cameras are capable of ISO speeds much faster than 400, and the image quality at this speed will remain quite good, it is faster than required for most shots.