This is the 999th most frequent Chinese word.
欲
Desire; want; wish.
In this sentence, '欲' conveys the intention or desire to extend one's vision further.
欲穷千里目，更上一层楼。
To extend your vision a thousand miles, ascend one story higher.
Here, '欲' implies an intention or strong inclination to say something that is restrained or deferred.
他欲言又止，似乎有心事。
He wanted to speak but hesitated, seemingly having something on his mind.
In this instance, '欲' is part of an idiomatic expression meaning 'desire' or 'aspiration' for improvement or progress.