The term "note" is used by perfumers to describe how quickly an individual scent is noticed in a blend. Notes are based on the volatility of the essential oil. Most essential oils are broken down into 3 main categories: Top, Middle, Base.
Top Notes (Head Notes) are the lightest in the blend. These are the essential oils that have the smallest molecules. These will be the scents that are immediately recognizable and will fade quickly, usually within a few minutes.
Middle Notes (Heart Notes) are the scents that become apparent after the Top Notes have faded. These notes form the heart of the blend and meld with the Base Notes to form the body of the blend. They will fade over time.
Base Notes last are the last to be noticeable in the blend. These are the oils with the largest molecules. They linger after the other notes fade and give the blend its long lasting fragrance. Base Notes are the oils with the largest molecules.
There are many recommendations for composing a balanced blend incorporating the different notes. These can be based on ratios or percentages. These rules are more important if you are creating perfume blends as opposed to therapeutic blends. Your nose is your best tool to use for formulating a perfume blend that is pleasing for you.