Holden imagines the plight of the ducks in a Central Park lake when the water freezes over. How will they survive? Will someone look after them? Or will they fly away?
In all three respects they resemble him: firstly he wonders how he will survive in a cold adult world. Secondly he meets a succession of adults who might look after him, help him. Thirdly, he has an idea of getting away from it all in some log cabin in Vermont when he can take no more of New York.
Also, after all those imaginings about the ducks in Central Park, Holden decides to go and take a look for himself and finds two things: firstly, not a 'single duck'. Maybe they have flown away - after all migration during the Winter months is a fact of Nature. Secondly, he finds the water is 'partly frozen and partly not frozen.' This shows Nature is not as hostile as he thinks. If the world is similarly not so hostile, Holden, like the ducks, will survive.
To Holden, he is like the ducks. His life at the moment is stuck, depressing, maybe even frozen like the lake the ducks live in. He compares himself to the ducks and he wants to know how they escape the winter, how they escape the cold. He wants to know what to do with his life. He wants to know where the ducks go when it's winter and it shows that he needs a hint to how he could go from where he currently is.