Visualising is creating a mental picture of something you want to create intentionally. Let's just say you want a new car, you'd picture the make of the car, the interior, the colour of the car, you'd see yourself sat in the car, you'd see yourself driving it, you'd hear the song on the radio, things like that. Daydreaming is a series of thoughts that occupies your attention like something you're worried about or something you're attached to, you zone out with daydreams. I hope this makes some sense.