. = current directory
.. = parent directory
../ = gets you one directory back.
Chain ../ as many times as necessary to go up 2 or more levels.
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For example this is directory tree
parent
|-sub_parent
| |-current
|   |-working.py <-(file you are in)
|-index.csv

You are in working.py and you want to read index.php, you can chani "../" like this.
data = pd.read_csv('../../index.csv')