What does "university ranking" mean for a graduate admissions committee? Many questions on this website ask how university rankings affect the future career in research and graduate admissions. But it is not clear for me what is actually meant by ranking. There are various rankings of universities and many of them are contradictory. 

Thus, in the context of graduate admissions what ranking does the admission committee have in their mind? How does the committee decide which is a high-ranked school and which is a low-ranked school?
In the context of graduate admissions, the “ranking” of universities by members of the committee is a purely subjective one, and depends heavily from place to place, from department to department, and from committee member to committee member. That's why you're not getting a simple silver-bullet answer to your question.

Now, most admission committee members will rather rely on their own past experience with students coming from various schools, and would probably go further and would not “rank” the same the different undergrad programs from a given university. They would typically consider things like:


how ready were past students from program X for our graduate courses? e.g., did they know and master well enough all the prerequisites for our courses
how well did these students do during courses? e.g. what sort of grades do they get (knowing that a top student from school X usually doesn't make it to the first half of your program, for example, is interesting to know for future candidates from this school)
how well did they perform in terms of research? e.g. school Y has students with excellent grades and educational record, but are they prepared for a wide-ranging professional experience that is a PhD?