At the end of the day is an idiomatic expression meaning “ultimately”, “in the end”, and it also often has a flavour of “after all”. For example:
Tip: See my list of the Most Common Mistakes in English. It will teach you how to avoid mistakes with commas, prepositions, irregular verbs, and much more.
By the way, if you haven’t read my guide on how to avoid the most common mistakes in English, make sure to check it out; it deals with similar topics.