What is and how to calculate quartic interpolation?

I was reading the gist on the reward function used in OpenAI Five, but I didn't understand the way they calculate health's reward.

This is what they state: Hero health is scored according to a quartic interpolation between 0 (dead) and 1 (full health) to force heavier weight when the agent is near dying.

I tried googling but didn't manage to find an easy enough explanation for me to understand. What exactly is quartic interpolation and how is it calculated?

I understand the normalization part of scoring it between 0 and 1, but how do they increase the weight of certain values?