What does this quote exactly mean?

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

"Success is a journey, not a destination".

Simply put:

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

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

आप don't get sent to Success.

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

the attainment of popularity या profit.

a person या 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 आप want the meaning the original लेखन is referring to.