How can you refer to directories and files at different levels of a directory tree?

"this to specify directories deeper in the directory tree from the current directory. For example (in Unix), ./app/data/data.txt would refer to the directory app in the current working directory, the directory data within that, and finally the file data.txt within that directory. We can also refer to files further up the directory tree using the “parent” directory symbol which is two periods, .. . For example, ../../system/data.txt would refer to a file two levels up in the subdirectory system ."