Think of scent notes like a story that develops over time, beginning, middle and end.
Top notes: The ones you'll smell first. They're instantly obvious when you pick up a candle and smell it cold/unlit. They'll be scents like citrus or herbs. They're light and fresh.
Mid or heart notes: You'll notice these after the candle has been lit a while. It's the main personality of the candle, if you don't like these, this candle scent isn't for you. You'll smell this for most of the burn, it's the balancing most well-rounded of the scent notes. This could be scents like florals, fruits spices or woods.
Base notes: These are the lingering scent notes, after the candle has been blown out your room will be left smelling of these. These add depth, warmth and complexity. They might be scents like amber, tonka, musks or woods.