what does nan mean in pandas code example

Example 1: how to fill missing values dataframe with mean

sub2['income'].fillna((sub2['income'].mean()), inplace=True)

Example 2: to detect if a data frame has nan values

> df.isnull().any().any()
True

Example 3: represent NaN with pandas in python

import pandas as pd

if pd.isnull(float("Nan")):
  print("Null Value.")