It produces an hash table (multiset) y which tallies the frequencies of occurrences of items in the list x, i.e., y_i is the number of times i appears in x, or is 0 if i doesn't appear in the list.
i1 : y = tally {1,2,3,a,b,1,2,a,1,2,{a,b},{a,b},a}
o1 = Tally{{a, b} => 2}
1 => 3
2 => 3
3 => 1
a => 3
b => 1
o1 : Tally