What does this quote exactly mean?

Ты can look at it in a couple of ways. One is that success shouldn't be something Ты reach and stop there, but to keep going. Another way to look at it is that success is about what Ты need to do, not about where it gets you. Hope that helps.

"Success is a journey, not a destination".

Simply put:

Success is something Ты have to travel a while and work hard to get, Ты just don't arrive there. Success is something that can continue или like a journey, be cut short, Ты just don't arrive there.

The results from when I asked Google:

des·ti·na·tion
ˌdestəˈnāSH(ə)n/
noun
1.
the place to which someone или something is going или being sent.

1.
being a place that people will make a special trip to visit.

Ты don't get sent to Success.

suc·cess
səkˈses/Submit
noun
the accomplishment of an aim или purpose.

the attainment of popularity или profit.

a person или thing that achieves desired aims

jour·ney
ˈjərnē/Submit
noun
1.
an act of traveling from one place to another.

a long and often difficult process of personal change and development.
1.
travel somewhere.

So technically Success could be both, but it fits the definition for Journey, so that it what it probably means.

All this is contingent on if Ты want the meaning the original Письмо is referring to.