Answer:
Observed light shifts to a longer wavelength.
They indicate receding galaxies.
The light source and observer move farther apart.
Step-by-step explanation:
Red light is the lowest frequency visible light, violet is the highest frequency (hence INFRA-red, as in below red and ULTRA-violet, as in above violet).
Redshift is an effect where we observe light (from space) that we'd otherwise expect to be higher frequency than observed.
Redshift is an effect very similar to Doppler's effect, where we hear lower (lower frequency) sounds when moving away from a sound source.
Because we're moving away from the light source (or the light source moves away from us). Similarily, when you're driving, you'll pass more cars going the opposite way than those going along with you.
So, when a galaxy recedes (and fast!) we'll observe the light frequency to be lower than the galaxy actually emits. Lower frequency = higher wavelength.
To conclude - any answer that relates to the light being lower frequency, or higher wavelength, or the observer and source moving apart are good answers, whereas those that suggest the opposite are bad answers.