Flatten a list of elements in Pandas DataFrame

Use explode:

df = df.explode('subGroups')

You can fix your output by following :

pd.DataFrame({'name':df.name.repeat(df.subGroups.str.len()),'subGroup':df.subGroups.sum()})
Out[364]: 
     name subGroup
0  groupA      123
0  groupA      456
1  groupB      aaa
1  groupB      bbb
1  groupB      ccc