To be clear, the poem in question is George Herbert's "The Pilgrimage".
The fourth stanza talks about investing a huge effort to climb a hill, with much hope. He manages to reach the top, but all he finds there is a lake of stale, "brackish" water. If the top should be a metaphor for success, it would mean that success is never really achievable. We can (and ought to) climb and strive, only to find out that it's the next hill that will fulfill our hopes.
