That is a function to return the translated version of the input.
Say the user is a spanish, so whatever you give parameter, like “Hello”, it will fetch from the translation .csv files (located in each app under app_name/translations/, check in erpnext app for a start).
It is _('Hello') in python, and __('Hello') in javascript.
You will have to import in python though. ( from frappe import _ )
Hope it helps 