Quotation marks are a way of separating out a word, a phrase, a quote attributed to somebody and written exactly as it is spoken. Quotation marks, known also as quotes or inverted commas, can be used for the titles of books, movies, works of art, etc.  Here is a dictionary definition:
quotation mark
noun
each of a set of punctuation marks, single (‘ ’) or double (“ ”), used either to mark the beginning and end of a title or quoted passage or to indicate that a word or phrase is regarded as slang or jargon or is being discussed rather than used within the sentence.

I love quotations because it is a joy to find thoughts one might have, beautifully expressed with much authority by someone recognized wiser than oneself. ~Marlene Dietrich

Quotation marks are used to show the beginning of the word or of a phrase to indicate that this word or phrase is in some way separate or should stand out from the text.  Quotation marks are either double “like this” or single ‘this’ – although you can choose either of these ways to mark a quotation, there are some rules that apply to their use. When quotation marks be inside other quotation marks, use single inside double quotation marks, or the other way around.
“I would love you to read ‘The Book Thief’. It is fantastic book.”

when quotation marks are used

Quotation marks are used to acknowledge someone else’s wording exactly such as
“If you tell the truth, you don’t have to remember anything.” Mark Twain

Or to show that something is an understatement, a cliché, some kind of slang or jargon:
The “intimate supper” was a dinner for  200 people.

Quotation marks also show in the body of a text, dialogue or direct speech
The weather was getting worse, the sky darkening.  John looked out the window. “Do you really think we should take the boat out today, Charles?” he said.

Quotation marks can sometimes be interchangeable with italics, especially when with titles of books, movies, etc.
“The Book Thief”, or The Book Thief, was nominated for many literature prizes

British English and American English differ slightly in the way that quotation marks are used, and a style guide will tell you how these uses and placements vary.