"Thank you very much."
The word לכן means "to you" for feminine, plural. It's the sort of phrase you might include in a speech of formal thank yous. In most every day situations, תודה רבה is quite polite. Literally, the difference is "many thanks" or "many thanks to you". But in practice, the only reason you would need to add an addressee is if it is unclear.
The phrase ברוך הבא means welcome, as in welcome to a space, like a home or a school or a museum or a city. Literally it means "blessed is the one who comes", but in modern Hebrew, it is also used without a religious connotation. The phrase אין בעד מה is an idiomatic way to say "you're welcome" as a reply to a phrase where someone says "thank you".