What and why is the small u in {u'key': value} printed out from the list of collections.Counter item

Question

I create a list of Counter item named calc_comp.

calc_comp = [sum((Counter(composition[aa]) for aa in 'ACDEDE'), Counter())]
print calc_comp
out>> [Counter({'H': 34, 'C': 24, 'O': 14, 'N': 6, 'S': 1})]

Then I append calc_comp into pandas DataFrame

df_M = pd.DataFrame()
df_M = df_M.append(pd.DataFrame({'comp': pd.Series(calc_comp)}), ignore_index=True)
print df_M

out>>
                                               comp
0  {u'H': 34, u'C': 24, u'S': 1, u'O': 14, u'N': 6}

What is the small u in front of each key of the output? Why is it printed out only at this point but never before?