What does it mean for an equation to express y in terms of x

Read the excerpt from gilgamesh: a new english version.gilgamesh backed away. he said,"how dreadful humbaba's face has become! it is changing into a thousand nightmarefaces, more horrible than i can bear.i feel haunted. i am too afraid to go on." §enkidu answered, "why, dear friend,do you speak like a coward? what you just saidis unworthy of you. it grieves my heartwe must not hesitate or retreat."