Count NaN values in a Pandas DataFrame
A DataFrame is a two-dimensional data structure, i.e., data is aligned in a tabular fashion in rows and columns. Each row has a unique identifier called an index. A DataFrame can have multiple columns, each of which can hold a different type of data. NaN, or "Not a Number", is a numeric data type that is used to represent missing data. NaN values are often generated when data is missing, or when there is an error in the data.