I don't quite understand the quotation marks here.

"Questions" is a variable, I assume that holds a numerical value. That variable is being concatenated with two strings. A string starts with a quotation, and ends at the next quotation; which is where you're being confused here. The opening bracket is a string of its own, it's concatenated with whatever value "questions" holds, which is then concatenated with the second string, ' questions left]'. After concatenation, questions will no longer be an expression, but rather one long string value.

var questions = 5,
questionsLeft = '[' + questions + ' questions left]';
console.log(questionsLeft); //this would return the string "[5 questions left]"

It may be more intuitive seeing it with string interpolation

var questions = 5,
questionsLeft = `[${questions} questions left]`;
console.log(questionsLeft); //this would return the string "[5 questions left]" as well

Another way to look at it could be to imagine the types

//Note: not actual code to be ran, just an example
var questions = Number,
questionsLeft = String + Number + String;
console.log(questionsLeft); //returns String