What does this command do?

cat /etc/passwd | tr ':' '\n' | sort | uniq -c | sort -rn | head

cat will output a file, the file /etc/passwd is the login file
tr (translate) will convert char to other chars
    convert ':' to '\n' (new line)
sort will sort the newly created lines
uniq -q wil remove repeated lines, -c option will provide a count
sort -rn sort in reverse number order
head will output the first 10 lines

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will output the 10 most command "fields" in /etc/passwd