Verily is an old word meaning truly, or really — used to press a statement home, the way we now say "honestly" or "I tell you." Its home is the King James Bible, where Jesus repeatedly says "Verily, verily, I say unto you," and that one phrase is why almost every English speaker recognises the word without ever using it. In living English it is effectively dead. When it appears today, it is doing one of two jobs: sounding deliberately biblical or antique (in a historical novel, a hymn, a sermon), or being funny — dropped into a modern sentence to mock its own grandness, as in "verily, the coffee machine is broken again." That joking use is common online and among people who like old words. It is an adverb and usually stands at the front of the sentence, followed by a comma. If you are writing normal English and want the meaning, you want truly, really, or indeed instead.