The French word ombré means "shaded." This trend refers to something that starts out one color and then transitions to another, or changes along a color gradient (i.e. hair, nails, cakes, dresses & fabrics etc.).
With hair it's also referred to as "dip-dyed" & it can be traditional colors (often with bleach) or more colorful rainbow ombré. Some examples are more subtle & some are bolder, some are just the tips with highlights & some make use of all the hair.
Here are some pretty examples I've seen recently of ombré hair.