It acts similiarly to Excel's INDEX function. It gives you the value from dataframe when you specify the array indices(row and column)
Usage
INDEX(array, row_num, column_num = 1)
Arguments
array
Which array/table should it use?
row_num
Which row should it return the value from?
column_num
Which column should it return the value from?
Value
In this example we get 3rd row and 2nd column from the dataframe. This function can return different classes numeric, character, logical etc. It depends on what is in array/dataframe.