Stack df1 on top of df2 (rows of df1 first, then df2) into one DataFrame, discarding their original indices and renumbering the result 0, 1, 2, ….

Input

df1 =
  name  score
0    A      1
1    B      2
df2 =
  name  score
0    C      3
1    D      4

Output

  name  score
0    A      1
1    B      2
2    C      3
3    D      4