This is the 2737th most frequent Russian word.
единственное
"Единственное" means "only" or "sole" in English.
Here, 'единственное' is used as a pronoun meaning 'the only one' or 'the only thing', referring to a unique or singular concern.
Единственное, что меня беспокоит, это погода.
The only thing that worries me is the weather.
Here, 'единственная' is used as an adjective agreeing in gender (feminine), meaning 'the only' or 'the one and only', to describe the noun 'книга' (book).
На полке лежала единственная книга о путешествиях.
There was a single book about travels on the shelf.
Here, 'единственный' is used as an adjective agreeing in gender (masculine), meaning 'only' or 'sole', describing the noun 'шанс' (chance).