Here's the breakdown:
* hath: This is the third-person singular present tense of the verb "have". It means "has".
* in the ram: This refers to the Green Knight, who is described as having a ram's head.
* his halve cours yronne: This means "his half course run", referring to the time allotted for the Green Knight's challenge.
Putting it all together, the phrase means "The Green Knight has half his allotted time run out.
In simpler terms: The Green Knight is halfway through the year he has given Sir Gawain to find him.
This line is significant because it creates a sense of urgency and impending doom for Gawain. He knows that time is running out, and he must find the Green Knight before the year is up.