uniq joins lines that are the same and next to each other into one line. With -c, it also writes how many lines it joined.

learner@tidepool:~$ cut -d' ' -f2 visits.log | sort | uniq -c
   2 ana
   1 ben
   1 chen

Sort first

uniq sees only the line just before. Lines that are the same but not next to each other are not joined:

sort puts the same lines next to each other. So always put sort before uniq.

Which is the most common?

This pipeline is a standard answer to "what happens most often?":

learner@tidepool:~$ cut -d' ' -f2 visits.log | sort | uniq -c | sort -rn
   2 ana
   1 chen
   1 ben

Add | head -n 3 for the top three.