System 32

Windows Note Pad is the program I make use of to write.
I have uninstalled the Office Word as it was too much for my use.
When 'open' menu of Note Pad is clicked the box 'Open'
comes on with 'Look In' and the folder 'system 32'.
A click on the down arrow next to the 'look in' and 'system 32' brings on an array of folders and 'my documents' is selected to open and work on an already saved folder.
I would very much like to know what this 'system32' is.
Please let me know how this system32 is used.
Thank you.