It is parameter axis in drop. It is same as axis=1. And it means you need remove columns from DataFrame which are specify in first parameter labels:
labels is omited most times.
Parameter axis can be removed if need remove row with index, because by default axis=0.
Parameter axis=1 is sometimes replaced by 1, because less text, but it is worse readable.
Sample:
import pandas as pd
df = pd.DataFrame({'label':[1,2,3],
'label1':[4,5,6],
'label2':[7,8,9]})
print (df)
label label1 label2
0 1 4 7
1 2 5 8
2 3 6 9
print (df.drop(['label'], 1))
label1 label2
0 4 7
1 5 8
2 6 9
#most commonly used
print (df.drop(['label'], axis=1))
label1 label2
0 4 7
1 5 8
2 6 9
print (df.drop(labels=['label'], axis=1))
label1 label2
0 4 7
1 5 8
2 6 9