Think of now as being the immediate present, the moment you are currently in. It’s part of the elusive concept of time, and each now, as it passes, becomes "then" even as it is replaced by a new now.
If someone calls to ask you what you are doing and you reply that you've been waiting tables but you hope to be a movie star and in fact you've saved almost enough bus fare to make it to Hollywood . . . they might interrupt you to explain: "I mean what are you doing right now." In which case you might reply that you're cooking some noodles. If you do something "every now and then," you don't do it all the time but every once in a while.