Year    Month   TAGS
2019    5   A, B
2019    5   A, C
2019    5   A
2019    5
2019    5   B, C, D
2019    5   C, E

A
B
A
C
A
B
C
D
C
E

check=[]

for j in range(len(test_df)):

  for i in range(len(test_df['TAGS'][j].split(', '))):

      check.append(test_df['TAGS'][j].split(', ')[i])