Contrast
Contrast is how strongly the colour of the text stands out from the colour behind it.
In a simple example
Dark brown words on cream paper are easy to read. Light grey words on white are hard to read.
Why it matters to you
If people cannot read your words in sunlight on a phone, they leave.
Do not confuse with
Contrast is about the difference between two colours, not about how bright or colourful each one is.